>> While I wholeheartedly agree this as a general concept, I find it tricky to accomplish in practice. Ianal, but afaik in general your employer owns the ip, and as such publishing it as oss requires explicit permission.

IANAL but if you give your co-worker a copy to run, you've just used the OSS license to be able to do that right? The co-worker now has legal rights granted by the license right? Including to redistribute your changes?

This all seems really silly since pushing changes upstream is by far the best way to be sure the changes are maintained. Not to mention legal uncertainty around maintaining an internal proprietary fork.