(Original thread title " Software to make iPac4 send Xbox360 inputs?" which I changed after I found the solution)
I know it's 4 years since I posted this, but I wanted to update it with my very successful solution (thanks to the genius of my partner, it astounds me the stuff she thinks of). The truth is the solution was reached not long after posting this, and I never updated the thread. I'm checking in again with a different question but I want to update this thread for everyone else's benefit.
My partner remembered me talking about knock-off XBox 360 gamepads you could buy on ebay - they were very cheap, and worked perfectly, except the only complaints people had were the physical build (trigger responsiveness, etc.) .
She said just buy 4 of them cheaply off eBay (about $12 each if I recall correctly), de-case them and solder the inputs directly - we're just using the circuit board of it, ignoring the physical properties of the gamepad. And you know what - it works great! I de-regulated my IPAC to just be inputs for the general menu buttons on my custom arcade buttons (such a sad job for a powerful piece of hardware), and the actual 4 player controls are each wired up to the de-cased internals of 4 knock-off XBox 360 gamepads. Each player has an arcade joystick that is mapped to the d-pad, and also 2 arcade buttons underneath the joypad, and 6 arcade buttons for actual gameplay. Might be overkill but it supports everything, even 4 player steam games. (And the analogue sticks are just doing nothing.)
The trick to soldering the bare wire to the contacts under the rubber buttons, is flux. Without flux, the solder just doesn't hold, it's a flat surface with no other way to get the wire to hold. But if you clean off the contacts, apply flux liberally to the wire and contact area, and then solder on with a tiny bit of solder, the connection is rock solid.
This is a setup that maps to any emulator (MAME, Retroarch...) and any Steam game (Duck Game, Knight Squad...) because while only some games support the keyboard, EVERYTHING supports XBox 360 gamepads.