There are good resources for historical handwriting -- palaeography* --, which is an important area of study for anyone dealing with old documents, but not much attention paid to modern handwriting.

*I have my eye on this book.

This sounds more or less like the topic I approached in my thesis, see
http://florian.hardwig.com/manuscribe/
Parts of my research, together with the work of others, were used by Ken Barber for the culture-specific stylistic sets in his Studio Lettering Collection:
http://www.houseind.com/fonts/studiolettering/fontfeatures

Regarding the diacritical marks, these fonts feature a form that looks like ó for ø (Danish), a tilde-like mark for the dieresis (Swedish, Finnish; also for doubles), a steeper accent for ć, ń, ó, ś, ź etc. (Polish; kreska, not acute), flat tildes (Spanish), a less angular, more breve-like caron for č, š, ž (Czech; háček) – among other things. They also have a proper double ew, and a ‘German u’ with hook: not exactly a diacritic, but a distinguishing mark (from the ‘n’) – a courtesy to the reader, as Ken put it.

Patterns that I have observed and that have not been incorporated in this font set include the ‘Swiss umlaut’ (horizontal bar instead of two dots), and the Portuguese habit to sometimes place the tilde more to the right.

Keep in mind that these preferences are all but set in stone. Some are true for handwriting only, others for lettering. Just because some letterforms (or accent placements) can be found repeatedly – in some scope of application, place and period – does not make them the default. They can help adding a local and vernacular flavour, but just as easily may look dated or odd, even to natives.

Florian: ó is often used for ø in Norwegian handwriting also.

Good to know, Frode! In the Studio Lettering fonts, this feature is only active for Danish, though.

Thank you for the link, Florian. Did you ever do a second run of posters? And are you ever going to publish the book in a large edition?

Source for vernacular Portuguese

James, here you can find examples and even vernacular fonts based on Brazilian Portuguese. Hope it helps.

BTW, this is an interesting thread.
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Portuguese tilde

Florian, what you call a habit (to dislocate the tilde to the right in Portuguese) is actually an error with interesting historical roots. Let me explain.

The origin of tilde is medieval Latin. It was an abbreviation to n/m (as "cũ" meaning "cum"). Later, tilde become used to indicate various vowel nasalization cases before finally being limited to four situations:

– short nasal "a" at the end of a word (lã). The sound needs ã because there is no final n in Portuguese, except for loan words.

– deep nasal diphtongs "ão", "ãe" and "õe", which resembles to late Latin aun, æn and œn.

Portuguese linguists discussed during 17th and 18th Centuries if tilde must be put over the first or the second vowel of these diphtongs. Between ~1700 and ~1820 the standard was to write "aõ" instead of "ão". This was later changed to the way it is used nowadays, but some people (mostly poorly educated ones) kept the old usage.

It's an uncommon error, but still found in rural zones or old street displays, as these you indicated. But it's plainly wrong, an error due to poor alphabetization.

This information is valid to Portugal and Brazil. I believe this was never an issue in other Portuguese-speaking countries as their colonisation was consolidated during 19th-20th Century (Angola, Moçambique, Cabo Verde) or got frozen since 16th (as Goa, Cantão and East Timor).

You can find book samples of aõ usage here.
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Spanish tilde

Tilde is not changed to a less curved shape in vernacular Spanish. What you probably saw were situations where the writer chose to add some "style" to the diacritic or just did not put much attention to it.

This is somewhat common both with Spanish tilde and Portuguese cedilla because they are unique diacritics in their classes.

In Spanish, ñ is the only tilde possibility. The other diacritic found in this language is acute. As they are quite different and there is no n+acute, people can write tilde even as a straight line and it remains easily recognizable as tilde.

The same with cedilla: ç is the only "under" diacritic used in Portuguese. So, even if you write cedilla as a vertical line or an unconnected comma, everybody will read it still as ç.

James, yes, I hope so. I will keep you updated.
Igor, thank you for the explanation, very interesting. Reminded me of the schoolfellow who couldn’t remember where to put the umlaut in the German diphtong äu, so he always wrote it on both: äü.

Florian: "a steeper accent for ć, é, ó, ź etc. (Polish; kropka, not acute)"
It ought to be: a steeper accent for ć, ń, ó, ś, ź. (Polish; KRESKA, not acute)
BTW Zdotabove has the alt form in UC: Z with horizontal bar, see my icon on the left.
Here: http://dl.dropbox.com/u/19437300/House9.png the dot above is unnecessary/wrong.

Janek, good catch, thank you. That was a typo – I have corrected it.
In the Studio Lettering fonts, the acute (on á or é) is also steeper, when choosing the Polish cultural set, although I understand that these characters are not native to the language.
Polish: http://www.houseind.com/t/4d7220
Default: http://www.houseind.com/t/338fc3

Here’s a plaque from Poznań that shows the Z with horizontal bar, as you described it, plus an Ł with a horizontal, one-sided bar (‘kreska nie ukośna’):
http://www.flickr.com/photos/hardwig/5873953317/

Florian, congratulations and thanks for the r in your Polish set. Many people forget (and I was just about to point it), that the r has a couple of variants in Latin handwritting.

second
And small update

Janek. Is there an equivalent to the "Ż" with the bar for upper case ź?

No. I am able to imagine that:

That looks good. My family abandoned the kreska from the ź in our surname when they left Poland in the 19th Century, but I'd like to start using it again. But I also sometimes write a crossed z as a purely decorative element and therefore have been making the mistake of a crossed and dotted ż in my Polish partner's name.

In Spanish, ñ is the only tilde possibility. The other diacritic found in this language is acute.

We use ü as well, but not very often (vergüenza, pingüino, lingüística). The dieresis indicates that the u must be pronounced (otherwise it is omitted, as in guitarra or guerra).