Is it polite/appropriate to tell a recruiter that I'll get in touch with them in several weeks? Their offer's really interesting, but right now I'm in charge of a large project and I don't want to leave it unfinished.

Of course. They're lucky you're interested at all.

It's ok and you should inform them about the matter
If they understand you'll know that this is the perfect job
And this can prove your loyalty to your projects

Very polite 😄

Maybe mention that to the recruiter? Sometimes they are looking people for some specific project and it you tell them that they might move along with someone else. However if they know you're interested but have something to complete first they usually understand. I've had almost all of our employees say something similar and it's a good sign usually (not a recruiter but cto)