It's been a few years since I cobbled this one together and I recall reading the IfIsEvil web page. I vaguely recall that at the time there was some limitation on how try_files worked that made it necessary to use ifs, but for the life of me I don't remember what.

The rtCamp configuration I linked to earlier makes use of try_files and says he has support for both supercache and domain mapping, so I think I'll migrate to that.