- feedjumbler, which was so broken to the naked eye I had to peek at Google's cached version to see that it would indeed have been what I wanted, and
- Blogdigger, which would let me setup a merged feed, and then deliver an empty feed. Update: As noted in the comments below, this is actually an expected result, due to an (at my time of visiting) undisclosed initial indexing delay.
Oh, well. Maybe I just had really bad search karma today. The real solution to the above problem still lies over at userscripts.org, where Jesse and Britt have been really busy doing other things lately. With a bit of luck, I'll be able to join the team to do some tweaking and feature additions myself in a while, such as creating aggregate RSS feeds of all comments to scripts from a particular author, which was what I set out to do above.
This is one of the things I really like about many of the really good projects on the web today -- the barrier to joining in to help out has felt much lower than it used to do a few years ago, at least to me. Prove yourself capable and worthy of trust, acquaint or befriend the people behind the web fronts and team up with them, for the good of the project. You also make a lot of really good friends with people who, much like yourself, want to improve the web and its tools.