Im confused as to why additional wiring would be necessary.
My trackball's mouse buttons are wired to player 2's 4 and 5 buttons. Making those player one and two start inside mame is as easy as changing the key assignment in mame to be mouse 1 and mouse 2.
You dont need to wire that button to anything but the trackball interface, unless Im missing something.
While running a rom, mame doesnt care if its a keyboard stroke or a mouse click. Just tell it what you want to do when the input is registered, and it will work.
If you need another key to be a shift key, you can wire that up seperately to the IPAC. It doesnt really matter if its button 1 or start 1 thats wired to the Start 1 terminal, you can define it to be whatever you want inside of mame.
Did any of that make sense? Wheres my coffee...