HDPE drum? Interesting concept and I can understand the reasoning behind it, but....

The teeth may take the impacts rather than them reaching the drum, but the drum still has to be strong enough to put up with the forces that'll be going through it. Regardless of how the teeth are mounted, those forces will transfer some way or another and, over time, will probably cause deformation of the drum.

Also, can't tell from the pics (on my phone) but would you be running the belt from the brushless directly onto the drum body? If so, the HDPE isn't going to enjoy the heat generated from friction.

Looks a compact, solid design overall though.