At my last job working for a small software company, I was able to get my boss to modify the employment contract such that a specific list of other projects (attached as an appendix to the contract) was excluded from the work product & IP ownership clauses. The idea was that if I wanted to work on a side project, I would clear it with him first, and then we would sign a quick amending agreement to the original contract which added the new side project to the list.

Major caveats: I haven't ever tested this method in an actual legal dispute. Also, the negotiation involved a shouting match in a crowded bar. YMMV.