Thanks to all the input from everyone.
I have what I think will be my CP design, although I welcome feedback and opinions.
Sorry for the crappy text version but I'm confident everyone can understand this

My planned button layout will be:
(p1b4) (p1b5) (p1b6) (p2b4) (p2b5) (p2b6)
(joy) (pause) (joy)
(p1b1) (p1b2) (p1b3) (p2b1) (p2b2) (p2b3)
The pause button in the middle will be my shifter, and I will shift buttons for the following functionality:
Pause Pause (duh)
Pause + p1b4 Insert Coin
Pause + p1b5 Player 1 Start
Pause + p1b6 Enter (select Game)
Pause + p2b4 Insert Coin
Pause + p2b5 Player 2 Start
Pause + p2b6 Exit (Quit Game)
The way I figure things, in most games you only use the first three player buttons (bottom row) at most, and so it should be fairly safe for one of the players to be using the shifting function (top row buttons) without the other player accidentally sending the wrong signal by inadvertantly pressing a button while the shift is down.
For this reason, I've also decided to make the exit command be part of the Player 2 buttons, so if you are playing pacman, wonderboy etc, there is no chance of acidentally exiting back to the front end.
The only times both players use all six game buttons are in the middle of a game of SF2 for example - hardly the time for adding extra coins!
So all in all, I think this layout works on minimising buttons while keeping accidental exiting etc to a minimum. What do people think?
Before I forget, this whole setup kind of hinges on being able to get button inserts/discs so that guests can understand iconically how everything works. I have already contacted MikeDeuce about getting the inserts. Hopefully he is still making them and I can get some. If not, it's either rethink the layout or use different colours for different buttons... and I really, really want all the buttons to be red translucent Happs!!

As for the seventh player button, I may or may not put these on the CP. As I've written in another topic, I still have the full romset for Mame 0.77 from a cab conversion from Nov 2003. One of the things I told myself was, for my next cab, to cut the number of games down to only the ones I like to play. So, the seventh player buttons will depend on whether I find enough Neo-Geo games to keep in the list to justify the extra buttons.
I don't even like street fighter - these extra buttons are purely for guests. I'm real old school for this stuff - Galaga, Wonderboy, Pacman, Bombjack etc.
Anyway, thanks for reading, and assisting. I'm going to have many more questions coming up on hardware, software etc and just generally support to justify the expenditure on something I don't need but SOOOO want. Long live MAME!