oh in answer to your question, an ipac has 8 button inputs for each player in addition to the coin / start buttons. So, two things can happen: the seven buttons next to the two main joysticks would wire to buttons 1 through 7, or one of the buttons in the 7-button layout would double up and they would wire to 1-6. For MK3 (looks like what they had in mind), you'd probably double the extra yellow button with the top middle button in the bank of six, so that the bottom middle would be block, the ones on the left and right be punch and kick, and the top middle and extra yellow one would be run. Then Neo Geo games could map the extra yellow and the other three bottom buttons as buttons 1 thru 4.
That leaves either one button each or two buttons each for random admin buttons, which it looks like he has on the top row. Since there's a trackball, he probably has an Optipac in there - that gives four more buttons that can be wired, so he has a total of 6 to 8 available admin buttons (looks like he uses 5 of them... why? why?).
The remaining buttons would end up having the same functionality in games as the first few buttons on the player 1 joystick, so they would just be doubled up with those wire connections to the ipac. Of course, this also means that people can reach up and smack those buttons while you're in an intense fighter match and make your character do stuff, and those buttons can't be unmapped for games that don't use them. The 4-player joystick in the middle would just wire in parallel with the left joystick.
Well, this is how I'd wire that panel to the IPAC-2:
1.) remove all controls and buttons
2.) start fire using existing control panel
3.) cook s'mores
4.) make new panel
5.) wire intelligent button configuration to ipac-2
6.) eat s'mores (eew, they got cold!)