coin shaped rfid tag
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?