You might consider asking in the context of a more general compensation discussion than just "Hey boss, can I work on OSS?" That way, if they say "No side projects", you'll say "OK, in lieu of the side projects, I'll need more money." It's easier to be sticklers for the stock agreements when there's absolutely no cost to the company to insist on the usual boilerplate, but minor concessions on the boilerplate are often easier than concessions on things which actually appear on the company's books.