Where I work, there's a fairly streamlined process for getting waivers for side projects, including open source contributions. Fill out a page-long form broadly describing the intended side project and send it to Legal. If approved (and I've never had one rejected), a couple of weeks later they send back two copies of a document signed by them saying that the employee promises not to use company time or resources for that project and in return they agree to disavow any interest in it. Sign and date both copies, send one back, keep the other. Done. Everyone knows where they stand.