I wanted to do this for Ludum Dare and some people gave me grief about it, even though it's legal by their rules.

My AGPLv3 code is here: https://gitlab.com/activated-onion/ld42-warmup

Here's a screenshot: https://social.freegamedev.net/photos/activated_onion/image/d2286632f9b14c30b5c3...

There's no gameplay, it's a graphics tech demo with physics. By LD rules I would make new art, sound, levels, and any game-specific code. The physics, input, file loading etc. low-level stuff would be reused.

Is that okay for OpenJam? I've gotten used to using my own tech and I don't feel like using anything else.

Yeah as long as there is no game play and it's just a starting example.  I think that's fine.

As far as I know, basecode is allowed here AND at LudumDare as well. The only rule at LD is that the basecode you're using has to be publicly available to all other contestants as well for free before the competition starts.