Instead of changing environment variables when $PWD changes, I went the other way: make it obvious when you're in a subshell by strapping $VIRTUAL_ENV onto $PS1. Again, simple shell scripts win[0][1].

I'm still interested in a cross-language approach, but I'm suspicious of anything which requires an explicit import step for rubygems that doesn't involve resolving the dependency tree. Unless Gemfile.lock is involved, it's going to need to copy Bundler's resolution algorithm, which seems fraught.

[0]: https://github.com/regularfry/gemsh [1]: https://github.com/regularfry/rv