I'm using the technique for finding related entries using a full-text index on mysql shown here.

Very slick.

However, it matches using title, keyword, and excerpt-- only the first of which I have used in the past. I will start using keywords now, but is there some simple way to at least get, say, the first 50 words of every existing entry into the db as an excerpt for that entry? This way it can find related material in my old entries, which it currently cannot...

I know MT can create one automatically, but in this case the excerpt must "really" exist in the database to be used for the full-text match...

Hmmm, I read the code, and I'm not sure. But maybe you are more familiar with MySQL select statements than me.
should do the same thing as the Excerpt set at 50 words. Can you use that in there instead?

Kristine