[Djur]

This is our team's custom mecanum code:

jaguarDrive((jX+jY)/2, (jY-jX)/2, -(jY-jX)/2, -(jX+jY)/2);

jX and jY are just the X and Y coordinates our joysticks read (-1 through 1).

And the jaguarDrive method:

public void jaguarDrive(double FL, double RL, double FR, double RR)
    {
        m_frontLeftMotor.set(FL);
        m_rearLeftMotor.set(RL);
        m_frontRightMotor.set(FR);
        m_rearRightMotor.set(RR);
    }

This is by far the simplest solution I've seen.