Last year, we used an Actuonix (formerly Firgelli) servo motor on our robot. A few inspectors were interested, but only because they hadn't seen it before. It plugs directly into the MR Servo controller.
We asked in the game forum if it was allowed. Here's the answer.
"
6v version with the RC control option to make sure it is compatible with the allowed servo controllers."
Here's the direct link on where to purchase them.
http://www.actuonix.com/RC-linear-servos-s/1853.htm
FTC 3491
FIX IT