There was some info about this in Moye's book, 'Type by Design'.
I recall testing it once, and it didn't come out very well. A lot of tweaking is necessary, so maybe doing it all by 'hand' from the beginning is not such a bad idea.

Find out the parts.
I attach this image, might help.
Best,

Ren

> try combining the scaling with a lopsided (horizontal-only)
> "automatic" weight reduction of the stems

That's basically the method described by Moye.

> try combining the scaling with a lopsided (horizontal-only)
> "automatic" weight reduction of the stems

I have tried this a few times and find that there is still so much cleanup afterward that it is of little use. The best use of this technique for me was to put the resulting stretched glyph on the mask layer and use it as a rough guide. Then copy pieces of your orginal font (mostly the joins) and position them over the joins on the stretched mask. You then have to draw new connecting lines or curves. I realize this is no big time saver but designing type isn't really ever a fast-food kind of experience. My advice is to just roll up your sleeves and do the work. What you gain in experience from doing it will only help you later. In type design, it is about the work so just enjoy it.

>I think just scaling it doesn't do it, because the
>verticals become wider,

To maintain a similar "color" to your condensed version you'd have to end up with wider vertical strokes. I don't think there's an easy way to do this.

What you could try it scaling it, then even out the differences between the horizonatal and vertical strokes by hand - ie the horizontals would become heavier, and the verticals lighter.

If you want to try some scaling effect, for characters with simple counters ("O") that might mean scaling horizontally the whole character, then scaling the counter "down" vertically and "up" horizontally (ie 95% * 105%).