That is possible, but is it needed? Depends.

Besides the obvious cost thing: How many action buttons do you want? Do you want to use the u360 shift button feature? What about mame's ability to map to multiple simultaneous keys? Many people feel that there shouldn't be too many or even any admin buttons on the CP.
IMO, the 8 buttons on the u360 is barely enough for a general CP (start, coin, 6 street fighter buttons + 7th neo-geo button sharing wiring with another button). While most of the FE navigation can be done with the joystick and action buttons (AFAIK), and most of mame's admin can be done with button combos, I am very against doing a simple combo or shift button for exiting mame. Once is too many times accidentally exiting mame in the heat of playing, and it's happened to me more than once with simple shift and simple combos.

Personally I'd like one or two

"admin buttons" in addition to the 8 from the u360, which makes the ipac a little overkill but ... I'd do it.
And while I'm not one of the "No Admin Button" people, I do agree the CP shouldn't be over crowded with admin buttons. Hidden buttons are okay, but IMO one pause button and one admin "shift" button (used with combos with coin, player, joystick and/or action buttons) are more than enough.
So I'd suggest use u360s for the action buttons (7 each? leaving one unused), and the encoder for the coin, start, and admin buttons. OTOH, if you were going with, say, four buttons per player, I'd say drop the encoder and do everything on the u360s.
My two cents, anyway.
