Hey everyone, sorry for coming to this thread late.
A while back I modified bulk-mail to handle HTML mail. This was based on work Mohan did for Sloan, but I made it a bit more generic at Don's request.
You can look at the proc "sweep" in tcl/bulk-mail-procs.tcl, in HEAD, to see how it was done; it could easily be done the same way in other places. Most of the work is done by procs in acs-tcl/tcl/mime-procs.tcl.
I had thought of one more thing I could do to make it even easier, but got distracted by some crisis or other and never got back to it. I will have to see if I can find any notes on what I was thinking of doing.