Just a note - in all places I've worked (BigCo's) they take a zero-tolerance approach to modifying the contract. It is their way or the highway. I guess they can afford to just say no and pass on you, rather than have loads of employees with their own unique snowflake contracts. Perhaps smaller companies are less strict, but IME big ones take zero shit in this regard.
Personally I don't have any hopes of such a feat at a big company but I can't stand to think it's not possible to have this anywhere.
From past interviews I've seen smaller companies are willing to compromises and are reasonable in their demands, but a real example of something that already exists and is in effect goes a long way to show you're not being overly pretentious (like I've been "accused" by previous managers before). Reason why I'm searching for what people actually have in their contracts right now.
Good luck.