I can't believe that nobody has yet chimed in with the following information:
As of the 2015 season, there IS an FRC-legal COTS electric lead-screw linear actuator available via FIRST Choice!
http://firstchoicebyandymark.com/fc15-037
As an FRC alumnus & mentor working at Bosch, I actually helped to coordinate the first donation of these motors by making sure that the documentation was detailed enough for the FRC crowd. That first donation was somewhat smallish by motor count, so they went pretty quickly; given the positive reaction to this first-year offering, however, I wouldn't be surprised to see the donation increase for the 2016 season.