I think this is great.
Thinking about the practicalities of joining in, I think it would help if you can consider the situation of new people, unfamiliar with the code, processes etc.
So, for example, expecting an 8 point job to be completed in 8 hours might be fine for the library maintainer who knows everything inside out, but way too quick for someone else who is getting up to speed.
I'm not sure how this can be made more attractive for new folk without slowing the project, but the prospect of putting in a day's work and then having the task taken off you would be discouraging.
Also, people do work at different rates, estimates are often optimistic compared to reality, so this could easily become a significant barrier to external devs. External devs will often not be full time, but working in their spare time. Etc.
One way to alleviate it would be to stretch the timescales for tasks not on the critical path, and to encourage new devs to go for these until they are fully up to speed.
Can anyone think of any other ways to alleviate this?