I might not have been a valuable employee, but I tried making a Plex clone in my free time when I was at Apple, asked if I could open source it, went all the way to the VP of tech to ask permission, just to be uncerimoniously told that it was not allowed, because they "really wanted me to be fully focused on Apple".

I'm not sure I agree at all that they'll say "ok" just because you're a valuable employee; I think you might have gotten lucky at your BigCorps.