Open sourcing a Unity game with assets

So, I have a goal to make and publish one (small, stupid, simple) game every ~Saturday. (I’ve made two, so far, thanks for asking.) I’ve started in Unity, because it’s convenient. My plan for each game was to have a free version with ads, a paid version without, but also have it open sourced (and if they’re willing to go to the trouble of removing the ads themselves, so be it).

Now, concurrently, I keep wandering into e.g. Humble Bundle, or Unity Asset Store sales, and picking up things I probably don’t need but maybe might use.

It has just occurred to me, though, that if I’m open sourcing these games – I kinda suspect the owners of these assets aren’t going to be happy with me just posting all their files on the internet. Pretty sure I can’t open source assets I’ve purchased.

Now, I could probably just use free assets and make note of where they came from, and everyone would PROBABLY be ok with that. But that significantly restricts the things I can use. (And it would be kindof a waste – you don’t want to know how many ancient untouched game-dev Humble Bundles I’m sitting on, haha.)

Is there some kind of workaround? Like, if I posted the whole project, but included a license stating that only my code was open sourced, and the rest could not be used in anything else, for instance? Anybody know any nice solutions?