Multipart templates?

Is it possible to create templates in RT where I specify the parts of a
MIME multipart message?

I’ve got a working HTML template by adding the Content-type header, but
the plain text conversion produced by RT is suboptimal (strips out
links, mashes tables). I’d rather provide a nicer plaintext version
myself if I can. I was imagining that this might work by me provider the
part boundary and then using it, or something along those lines…

Has anyone had any success with this?