You might try contacting them, indicating that you are passionate about their project, and would they be interested in hiring you as a 1099 for six months (or 20 hrs/week or whatever) to implement {feature}. I'm beginning to realize that while everyone (workers, businesses) have a fairly one-track assumption of what employment/work would look like, a lot of times businesses are open to non-traditional ideas. A lot of places are fine with part-time if they know they can count on you to get stuff done. PMs seem to value predictability over throughput. Also I think the trend to remote work will help the process of being able work on just a piece of a project.

Anyway, you might as well give it a try, see what happens.