I am seeing this question now only. Many may have read this slashdot article but if you have not see this: What Do I Do About My Ex-Employer Stealing My Free Code?
Though the slasdot original post is not directly related to this question many of the replies are.
Good that you are thinking of coming up with some formal rules. Here are some of the excerpts from the link I have quoted above, which are experiences of fellow programmers with their companies due to policies regarding contribution to open-source projects. There are lots of nice and eye opening answers but quoting 3 of them:
It's so standard that you should always ask for permission from your employer before writing and releasing open source software - you might not have the right to do so, even if the software is not related to the business of your employer and even if developed in your spare time; the language in my contract is unambiguous about that.
Assuming that in this case the permission to write and release open source software was implicit, it still does not mean the company has lost it's control of it's intellectual property - they can always dual-license it under a proprietary license. They can't "take back" the already released GPL software, and they can't grab any contribution of 3rd parties to that lineage, but they can chose to develop the original codebase in an entirely closed source fashion - it's theirs.
It is possible to negotiate with a company to preserve your ownership of your own personal pursuits but you must be proactive and generally have leverage (In my case I was holding up a merger with an entire IT/dev department. Your average shmo only has the desire of the company to have them work there). Also expect that the/any company will do whatever they can to own everything you are and do so presume you are screwed and read any documentation you are asked to sign with that intent in mind.
learn what MIT licensing means... they're entirely free to distribute under whatever license they choose as long as they cite you. They're not stealing it, they're using it under the free license you provided it under.
As for the GPL – yes, you need to get a lawyer there, that is indeed a violation. Of course – if you coded this GPL code on their time, it's their copyright anyway, and they're free to use it any way they see fit.