Fair use is built into copyright law and their EULA doesn't make it just go away. Remember Galoob vs Nintendo regarding Game Genie. Folks have the legal right to change the bits in software on their devices. Also, tying acceptance of an EULA to simply playing a machine .. that would be interesting to see in court. Remember, WMS had those FBI warning stickers in the backbox that said essentially the same thing, and everyone ignored them. If I buy a game secondhand and play it, I didn't legally agree to diddly-squat.
The problem is this: "and/or losing access to Stern Pinball’s online game network"
If they try to pull PSN or Xbox Live bullshit and force users to subscribe to said network get normal software updates (where they can track and ban you), that will be the end of Stern for me. If I want to, say, make my Metallica play folk music, that is still my legal right.
Streaming stuff will be an interesting fight.. I can see the desire to block streaming of modified 3rd party content (Metallica might get pissed if videos of folk Metallica proliferate) and such streams may not have strong fair-use protections.