Interesting stuff on ES re RFID keyswitches for bikes ... looks like there may be some getting made up soon for mod use :
Endless-sphere.com • View topic - RFID Keyswitch
Endless-sphere.com • View topic - RFID Keyswitch
If you want a cheap starting point try this, runs of 12v and has a few spare tags Security RFID Proximity Entry Door Lock Access Control System 10 Keys Free Shipping! - US$9.99