Heads up: I tried the subformula bit first, then had to resort to the caveat.  I think I made a note somewhere in http://github.com/chdiza/muttrevamp about it and why I thought it wasn't working.

The problem seemed to be that the relevant docfile is a single html file that is either fetched straight up or gzipped (without being put into a directory before being gzipped).  That was causing fetching problems.  If I *locally* put the html file into a dir then gzipped the dir, I could do the subformula bit OK.