If you don't want to make a new control panel, you could add a pair of DB-9s and use 2 pairs of 2600 paddles.
It'd pr'ly work better than rotating sticks anyways, IMO.
Except that you wouldn't have any interface to the computer for them.
The best handheld workaround that i know of is to interface to an analog stick, and use similar pots (usually 100k) mounted in small Radio Shack project boxes with a single button on each, out the side.
They will show up as two analog joysticks with multiple buttons.
You can then map the buttons/axes to the appropriate controls within MAME.
You could also mount the pots directly into the CP somewhere, and get an arcade cabinet version of this.