Is RFID the right solution
This is probably going to sound all wrong but my cats are micro-chipped and they find it easy getting in and out of the chip-detecting cat-flap. So here's where I redeem myself, find a source of the chips and attack the problem that way. I'd be guessing but I can't see them being big prices. Apparently, they are the size of a piece of rice so fitting them in a shoe is going to be a minor problem.

It looks like the MCRF355 is a good candidate for the electronics heart. It's likely though that you'll be using 13.56MHz rather than UHF. Yes, they need to be passive to keep cost down and be unobtrusive. MCRF200 runs between 100kHz and 400kHz and maybe a decent solution too. This link also covers the MCRF250.

Humans too