But the thing that sticks out the most to me is how will the spinner without the wheel get in the way of normal play? It looks like it will be on the wrists.
Well I tried it out on a full size paper mock up and it seemed ok, as your wrists kind of come in at an angle from the sides of your body, so mananges to miss the spinner by an inch or so.
How do the wheels attach? Ive never seen removable wheels and I like it! Do you have a link to where one can get them?
Groovy Game Gear 
I would think about adding an extra button, 7 each, for the neogeo layout. It comes in handy when playing some emulators.
I thought about that but I never really played any NeoGeo, and I thought 6 would be enough. Seven might make it look a little too crowded too.
Use a high-lip trackball, not the high-ball.
I already have bought the 3" ball from Ultimarc - is that the high-lip one? Too late to change now anyway!

Add some coin buttons, you can wire a switch to disable them if you just want the coin at some time. You can also hide them or use smaller buttons to make them less noticeable.
I'm going to use the coin door and have a bunch of coins in a jar handy

I think I'll wire up coin inserts to the shift buttons too.
Any thoughts on pinball buttons?
Never really was into pinball either!

The CP is to be swappable as I'll make some other dedicated panels sometime in the future. As it currently stands if I add pinball buttons it'll be a PITA to swap panels.
Also, if you are limiting the buttons because of the 8 offered by the U360's, you still have 3 from the trackball and possibly 3 more from each TT2. Mame can recognize those just as easily as the arcade pushbuttons, so don't be concerned with that.
Ah didn't know about the buttons for the trackball and TT2.. may come in handy, thanks!

I also have an IPAC4 handy, so I may use that. Might come in handy when I want to map buttons for C64 emulators to keys such as the C= key, space bar, run-stop etc.
Thanks for your feedback!