No, the Frontline controller could not be easily emulated as a spinner. Soembody would have to do a big funky hack. The reason it's emulated as a joystick is because it really IS a joystick. Under the panel, there's 4 microswitches. The rotary knob on top rotates an actuator that cycles through the standard 8 directions, pressing one switch each for U, D, L & R, and pressing two switches to get the diagonals. Electronically, it is exactly the same as a joystick. So MAME's emulation on this one is dead on.
Which doesn't help people with no Frontline controller. A hack for this is on the list of proposed features for PowerMAME, but who knows if or when it'll get done.