Hey All--
The Issaquah Robotics Society (IRS team 1318) created a mecanum drive last year that supported PID with encoders and driver-oriented steering with a gyro. It is pretty modular, so it should be fairly easy to incorporate into a design. At the very least, the equations are all there.
Here is the code on sourceforge:
https://sourceforge.net/p/logomotionreal/code/3/tree/
The critical aspects of the code (ie PID, kinematics) are tested under JUnit, and are known to work.
Hope this helps!