I worked at Centaur last month, and "faced" with same jumpers problem.

Repaired SIA and S&T boards, and there was not practical with applying the necessary jumpers,
but the solution is VERY simple.

Instead jumper I made modification and put a 100k resistor - S&T works "With or without you"
Mean on SIA board ! )

Yes, it was pretty simple... little knowledge about high-impedance inputs, mixing reverb (worked a lot
on guitar amps construction

With Bally pinball machines have worked at least, but here's my small contribution