Ryan, thanks for your input!
I was under the impression Silvercar required you to take a shuttle there, is this not the case anymore? Or maybe not an issue for you?
SFO has the airtrain/car pez dispenser, but I never need to rent a car at SFO.
At airports with big on-site car vending machines, you're not going to generally be able to do that at any scale without being arrested, and you'll probably be inferior to my current experience of walking across to the car vending area gold stalls, going to the number which was emailed to me, picking up my car, and driving off.
It might be more of an issue at smaller airports with no onsite cars, or where at certain hours all the shuttles are infrequent.