1675's mecanum code this year may be able to perform what you describe...
(I admit i have not seen almost any of the driver practice, as I was sick for the last couple days of build
) but it may come out as more of an forward arc. I'm not sure.
What our algorithm does is get velocity and angle from one stick, and rotation from another, then calculates the PWM value of each individual motor taking those 3 variables into account. You can see our algorithm here:
http://team1675.org/forum/viewtopic.php?p=1352#1352
Most of the calculations are not shown (ex: getting the velocity and angle, scaling the rotation value), but the key lines are
Code:
MotorOutput = (((WORD)mySin( Heading ) * (WORD)Speed) >> 8) + ((256 - Speed) >> 1);
MotorOutput += Rotation;
This is called for each motor. It first sets the value of each motor based on velocity and angle, then adds/subtracts the rotation value from each wheel.
This should create a spinning effect, but while moving forward, as I said, I am not sure, it may come out as an arc.