“RFIS” tag

I am be very interested to see how this works out.

I am currently making an RFID controlled lock for a door and I know from my work on that,that RFID have range limitations(depending on what type of reader and chip you use) so the position of both reader and chip may be very important especially because you're planning on using it on an unpredictable living thing. That might be a big challenge and may require more than one RFID reader.

As far as a locking mechanism you can make one really cheap by taking apart an old cd drive and using the slots drive system.

Won't help - RFID tags all have a unique number on them. And when I say unique I mean 64 bits unique, maybe more. This is 'more atoms than the universe has' unique. They keep a list of the people who are allowed to use the elevator and know what numbers they have. So if your tag hasn't been entered into the system you can't use it. Unless you could get your hands on one of the 'masters' tags and copy the number. But if you can do that just use that tag. Theft, sure, but you're already asking how to elegantly trespass so I'd assume you're okay with theft?