Finally figured out the keyboard encodings for the arcade cab. Below are the encodings, along w/ the shifted keys (misc actions in parens):

P1S = 1
P1C = 5
P1U = Up
P1L = Left
P1D = Dwn
P1R = Rght
P1B1 = Ctrl
P1B2 = Opt
P1B3 = Spc
P1B4 = Shft
P1B5 = Z
P1B6 = X

P2S = 2
P2C = 6
P2U = R
P2L = D
P2D = F
P2R = G
P2B1 = A
P2B2 = S
P2B3 = Q
P2B4 = W
P2B5 = I
P2B6 = K

Shifted keys:

P1S = <SH>
P1B1 = F3 (reset)
P1B2 = Tab (config)
P1B3 = ` (adjust)
P1B4 = F11 (fps)
P1B5 = \ (ffwd)
P1B6 = ] (fullscrn)

P2S = 5
P2B1 = O
P2B2 = K
P2B3 = A
P2B4 = Y
P2B5 = Ent
P2B6 = Esc (quit)

For the most part this set up works really well, as there is no conficts w/ buttons and shouldn't be any ghosting. The only real problem is fast forward. For a few emulators I need to change the keyboard bindings on the Mac to make the different keystrokes work w/ the same key across emulators. Unfortunatley in MAME it won't recognize "\" while playing the game to fast forward. Not a huge deal, in the grand scheme of things.