I'm not sure how things have been done in the past, but as of last week I'm in charge of the document production team and have access to editors timesheets.
I'd also suggest that you are overestimating the variance of speed (holding quality of the product constant) of workers who stick with the team.
Finally, tasks aren't always as clearcut as I think you imagine (not every task is, go convert this document and get it back to me) so that would complicate paying in a non-hourly fashion. Additionally clear cut tasks might vary in difficulty—converting one document might be easier then another—which means assigning a dollar figure isn't a trivial task and comes with it's own costs.
The Singularity Institute has recently made the transition to a LaTeX based document production workflow for its publications and republished its existing research papers. However, there is still much work to be done and we need more remote LaTeX editors. Some projects currently in the queue include converting both The Sequences and Facing the Singularity into LaTeX based books.
As with other remote positions, pay is hourly and starts at $14/hr but will increase if you produce a good product.
Perks: