If there was only a way to do drm with open source
Any time you think drm is the solution to your problem, you're solving the wrong problem.
Afaik drm is the only way to do paid licenses. Not root kit shit but closed source requiring a license key.
Ente's idea above sounds good.
goatpig and myself have independently come to the conclusion that a form of crowdfunding is a possible way to fund continuing software development under open source licensing.
So, I think there are alot of better options than some kind of signed/DRM closed source model. That's near enough the worst option to me; having published code (and better yet, re-producible builds) are the best way to implement software that manages something this crucial.