Alpha version of RarMount can be found here:
http://www.jackjson.se/rarmount/RarMountAlpha.zip
(the file is big due to an included FAT32 header file)
It's partly based on and-81's RarPlayer plugin. (Thx!)
A ReadMe is included in the archive.
From the ReadMe:
What it can do
**************
- Mount uncompressed RAR archives!
- Seemless intergration with Media Portal
What it can't do
****************
- The mounted RAR archive(s) may only contain ONE file,
and that file must be smaller than 3,98 GB. I.E: No dvd imgs yet.
/Jack