On an SS, I guess you'd have to choose your playfield based on whether or not they used the entire solenoid matrix... a lot of the earlier SS games didn't max out on solenoids compared to what the board could do.
Or, maybe you could remove a solenoid someplace else... or better yet, make them nonCPU controlled. Switch closes solenoid fires, no intelligence. That wouldn't be hard to do, just tap into the power line, I think. I know some of the earliest SS pins had solenoids like that.