Quoted from Zablon:

You are missing part of the picture. Pretty much anything can be hacked given the want and time. Stern is more susceptible because they are the biggest and put out the most games (on a standard platform). They reach the largest audience, so most likely to have people with that skill, time, and desire.
Of course, that is the key, desire. There needs to be a reason first. Most of it revolves around the endless band games. A few things with toppers, but in the grand scheme of things much of the 'mods' can be done without any software modification. The other smaller companies? frankly not a lot of reason to hack the software at this point, and since they are lower numbers, less likely to happen at all.

Quoted from Zablon:

Which part are you asking? If JJP can be hacked? Most likely. Is it worth it to most? Probably not. Unless someone really good is a huge pinball fan and sees a need or thinks they can make money off it. If millions of people bought pinball machines and had them in their home at their disposal 24/7, it would get done by someone, probably just to see if they can. And no, I'm not one who could do it or I already would have tried.
If you are referring to this 'mods can be done without software modification', I was referring to normal mods that require nothing more than power, not integrated mods. That being said, I'm sure there are some people around who COULD do it, if they wanted to. One of the first questions I asked on this site was if anyone was making custom images, and you could see it was a touchy subject.