Ben,

One of the major problems, as I see it, is that doing this at the printer requires doing it once per head (unless I'm missing something). For many of my projects I will be needing all 4 heads.

Will you be able to handle the extra mass of four pellet extruders that are moving in odd ways? I'm not trying to discourage the idea, but I can see a lot of problems.

A dedicated filament extruder costs under $800 for the fully completed ExtrusionBot or under $300 for the kit Filabot. These are easier to set up under ideal conditions such that the filament is spooled and comes out at a constant rate. They seem to be able to make about a kg a day according to their advertising, but I haven't looked much yet at real user testimony.