I would limit the problem domain so that delegation is easier.
1) submit unit tests in format XYZ
2) tests pass
3) receive code
Generate high level reports off of submitted code (kloc, cyclomatic, whathavs).
Another one would be
Design the urls for a REST interface and the JSON responses, worker implements code to support REST using framework A/B/C.
Or, implement algorithm in paper FOO using Python, Julia or Ocaml, should be able to read this dataset...
It has to be fairly constrained or else it is hard to specify and complete projects. They should come back within the same order of magnitude that one would complete them in anyway w/o either party being overworked.
One could also, instead of money, implement it as a sort of quid-pro-quo, I see a simple task (for me) that I could knock out, this gives me credits to spend on other projects. Virtual currency of code.
Also, licensing is an issue. I wouldn't want to work on code I couldn't keep. This just seems wrong to me. The only way I am ok with it, is if the rates are really high and that doesn't seem amenable to a site like this.