There's the possibility that you may have to provide a code repository without the art assets. The code will be available to browse and adapt, but people won't

There's the possibility that you may have to provide a code repository without the art assets. The code will be available to browse and adapt, but people won't be able to build and run a working game from it. I recall Id distributed some of their games this way--you had to go buy a retail copy to get the WAD files to actually play the game. Otherwise, you'll have to rely on documentation that explicitly states which files are available under what licenses, and trust people to behave. This may not be an option for assets that you do not actually own. A middle route might be to provide public-domain low-fidelity placeholder assets along with the FOSS version. If you are able to build a community around your project, you might get them to build these assets for you.