> This has been done, by several people. The problem is that the resulting packages don't fit with Debian policy, and in a few cases result in file collision

Then, by definition, it has not been done - it has been attempted and the problems proved hard.

I think the best way to go about it is leave the Debian Ruby subsystem alone and either go the virtualenv route or install it from the source tarball and place it in /usr/local or /opt