I finally figured it out. Inside "..." is an flink whose continuation function references x. srv.arc's default threshold for harvest-fnids is very large, so my memory was getting completely consumed before any fnids could be harvested; hence the references to each x were lingering, and mzscheme couldn't consider them garbage.
My workaround is to have foo make a call to harvest-fnids with a calculated threshold much lower than the default, and now memory consumption isn't getting out of hand. Thanks for everyone's help!
