There seems to be a lot of fuss going on here over mostly nothing. This issue is very simple. There is no gray area.
If you are making something that will not redistribute someone else's IP, then you do not need to worry about anything. You are legally safe, and this applies across the board in everything, not just pinball games.
If you are making something that will be redistributing someone else's IP, then you need to get permission from the IP owner. It's a simple thing to understand, and again it applies to everything, not just pinball games.
PPS & FAST have worked out an agreement that offers people a path to use Williams/Bally IP in their pinball game designs. So if you want to redistribute Williams/Bally IP in your pinball game design, they have a path that is easy for you to take. It's not the only path, as evident by MMR and BOP2.
Again if what you are distributing is your own work and doesn't include anyone else's IP, then you are free to do whatever you want with it. Also, if you are not distributing anything, and just making alterations to a game you own, then you are safe and need not worry.
Several of the PROC folks are making some really great designs. Some are "re-themes" where they are making something new, not using existing Williams/Bally IP. Some could be classified as "2.0" games where they are using Williams/Bally IP. Most of them appear to be one-off designs they are doing for themselves. So they likely won't run into any problems since they aren't going to be distributing anything. If I'm wrong and they do wish to redistribute Williams/Bally IP, then they'll need to work out a deal with PPS and/or WMS. A few of them have much bigger IP issues to deal with than the Williams/Bally one (doing TV show or movie themes). I don't think any of them need to stop what they are doing. I really hope they don't, because it's very inspiring and awesome work.
I personally own 2 PROC boards, a bunch of driver boards, and 2 of Mark's System 11 "adapter" boards. I am using one setup in a High Speed machine in order to experiment while I develop my C/C++ framework for making pinball games (something akin to the python stuff they have already, but I not a python fan so...). I have several System 11 machines, so I'll use the other in one of those once I have my framework done to a level that I can make something worthwhile on it. I also made a small contribution to the PROC software to help it work with the PDLED boards.
I've also Helped Ben Heck with propeller code for the sound driver in AMH, and I am working with Parker (board (co)designer for PinHeck and designer of his own "Tommy" board) to make software for his board. I'll be helping Aaron with software for FAST as well.
I'm helping all of these folks because I think they are all doing great work to advance the pinball market. I see no reason to "pick sides". I'll continue to use and work with all of them as much as I am able.
FYI, I intend to have my C/C++ framework operate with PROC, FAST, Tommy, PinHeck, and any other hardware that comes along and allows it. It's intended to be at a higher level, and will interface with the various drivers for each piece of hardware. I also intend for it to be open source.
Anyway, this post help in some way...