I wouldn't say that it's "completely" not worth it; it can be handy early on to quickly build up your seed stockpile with your first few crops. This means you need fewer seeds to start with, which saves you points you can invest in beer, which gets you free barrels. One unit of potash nets you about 8 extra plants in your harvest, which means about 12 extra seeds (and you can get double that if you time it right to get two crops in a single season). Even if it weren't for the beer barrel exploit, buying an empty barrel at startup and using that piece of wood you save to get potash would be a pretty good deal.
Once you get plenty of seeds in, though, you are probably better off just using mass farm labor.
Currently, I think the most wasteful part of the wood process is in making the better grades of glass; not only do you have to burn wood to make potash, you also have to use up charcoal to produce pearlash and again to make the glass itself. Silver ore is everywhere and takes way less wood to process, but is more valuable than clear glass (and just as valuable as crystal glass, which takes rare rock crystals).
As for the original post's suggestion... I'm not sure where it sits from a realism standpoint. The charcoal-making process might have an effect on the kind of ash produced which could make it difficult to extract potash, although really I have no idea how the particulars work.
[ August 26, 2006: Message edited by: RPB ]
[ August 26, 2006: Message edited by: RPB ]