I'm not aware of anybody doing such a mod previously, but it's certainly possible and should be fairly simple. I'm not aware of any side effects as long as you use a power wire that's hot only with the ignition on to prevent battery drain. I'd be interested in such a mod as well, since I always use cruise control even in town to help minimize accidental speeding and to get slightly better fuel economy. With the wiring diagrams from the
EL section of the FSM, you should be able to find all of the required information on performing this mod fairly quickly.