Just tried my IIDX controller hooked up through a Radioshack USB adapter and the results are as follows:
Turntable CW: Up
Turntable CCW: Down
Start: 10
Select: 9
BB1: 7
BB2: 8
BB3: 5
WB1: 4
WB2: 3
WB3: 2
WB4: Left
All the buttons register so I assume that it will work fine in MAME but there is the fact that the Turntable and the 4th white button are direction on the axis that may or may not screw with things...
PS, I tried to make the labeling as self explanatory as posible but if there's any confusion just let me know and I'll clarify.