Here's another page that allows you to drag and drop your own documents onto the page and see what the ligature substitutions do with your own text.
http://www.yourphotocard.com/Ascanius/Historical.htm
Again it supports advanced features that are only currently supported in Firefox 4 beta.

Except that JUnicode is like 1MB, so it takes a good 10—15 seconds for the font to show up ...

I wouldn't have used JUnicode except that its license allowed me to make the modifications required and it had all but one of the ligatures I needed. The font used is irrelevant to the process. The font could be considerably optimized but that has not been done as it's the effect that's important right now.

Though I greatly appreciate the forward moving effort here, the bottom line is that the resulting typography, it's effect if you wish, is not looking so good for the reader.

Well, I would hope you're not presuming to speak for all readers. To be able to take a regular text document and without losing the ability to spell-check and without having to type with special characters such as the long-s and ligatures directly, allows for great flexibility. The intention was to reproduce documents exactly as they were produced in the 18th century. I feel very confident I have achieved this goal. Almost all of the MUFI fonts have the required ligatures; which one is used is entirely irrelevant to me.

The intention was to reproduce documents exactly as they were produced in the 18th century. I feel very confident I have achieved this goal.

With drop shadows?

What? You're telling me they didn't have browsers in the 18th century? That was just a little show in the title. I think some of us are missing the point. The point is that there is a significant percentage of mankind's printed works (from the beginning of the printing press to early 19th century) which was produced in this manner. 33 million pages from the Eighteenth Century Collections Online (ECCO parts I and II) and another million pages in the Burney Collection of newspapers to quote two sources only. To be able to see this exactly as it was printed and then switch to a format that is easier for the modern reader (dberlow excepted) to enjoy is, for me, a huge bonus.