I know that this game does exist, so I'm aware there is a little bit of leeway for licensed ttrpg games.

I was wondering if it's possible (from a legal standpoint) to publish a toolbox system aimed to a specific genre, that is a standalone product itself, and then releasing free license specific "setting guides" on a side for the players interested to play with the original game in a licensed world.

e.g. The published game is a narrative game aimed to emulate scheming nobles in a generic fantasy world, with a big rotating cast of characters and a very high lethality. The free "setting guide" is an online pdf completely free to download for the world of A song of Ice and Fire, with reference to places, characters, names and how to use the rules of the actual system to play in Westeros (and not an "almost Westeros" with all the serial numbers filed off).