To Markrvp - just want to clarify that the information proved by no-one (that sounds weird) is accurate, but ONLY for the I-PAC line of encoders. Their shift function works on key release, so having a pause input activate a shift function is very feasible (and recommended).
On the KeyWiz, Shazaaam! is a separate input. You could program it to send a keycode also (like the I-PAC), see
http://www.mameworld.net/tigerheli/encoder/shiftkeys.htm where I talk about I-PAC compatibility mode, but the pause input would be a bad one to send, for the reasons you mentioned since the KeyWiz in this mode operates on the initial keypress.
I use the following buttons (besides start and coin) with the KeyWiz -
Shazaaam!
Pause.
Escape - I have a dedicated input for this, but accidentally quit a game once, so in MAME I re-mapped UI cancel to "=" and now press Shazaaam! plus this button to Escape.
I have Tab and Enter mapped to the pinball flippers and use them a lot for config, but, but if your software was already configured you wouldn't need to, or you could use Shazaaam! functions.
I use F3 (ShazaaaM! plus P2 Button 3) alot but wouldn't want a dedicated button for it.
Note also that by default (without re-programming) you can use Shazaaam! combinations to access the following keys (among others).
ESC - Shazaaam! plus P1B4 or P2B4
Pause - Shazaaam! plus P1B5
Enter - Shazaaam! plus P1B1 or P2B1
Tab - Shazaaam! plus P1B2
PgDown - Shazaaam! plus P1B3
PgUp - Shazaaam! plus P1B6