I like the idea of having the slightly more productive people being able to push content at a higher rate. Those of us writing novels are churning out WAY more than 3500 words every month, and people who want to sit back and relax through tricky stuff can.
What about this:
We have submit days on Saturdays, but to sign up for Saturday submission, you have to have your name on the list (which opens one week before the submission date) by Thursday (to keep people from trying to last-minute sign up for submissions). Moderators choose when we have enough people signed up (10 people max?), and people who haven't submitted as much get preference (if I've been turning in chapters weekly, and Raethe decides to turn in a short story after 3 weeks of nothing, she should get preference); people with relatively similar # of submissions would by FCFS.
Edit:
I included a handy diagram ^^
if you want to submit on a Saturday of a particular color, you have to put your name on the list during a day with that colored line on it.