Well, I suppose one way to go would be to put the interface that actually communicates with game/modified game code as a wrapper, then separate anything that could be perceived as derivative of game's code or data, and finally write the rest of the mod's code as a library that communicates just with your wrapper.

As such, the library could be licensed as any independent open source code could, and the company would have no more right on it that it would on any form of open source library code from anywhere else. Curious how would it end up if you decide to sell the library code itself tho - bet they'd want to take it down but which licence is broken by make a free connection/wrapper code to a paid service, etc.

It may not be a viable option for every mod tho.

Funnily, this was part of the discussion I had with another modder not long ago, on wether or how to make GPL apply, tho before this circus started. But as many others, I'd considered it to be too much of a hassle.