A nice POP, best of luck with the ESC hacking.

I don't think direct drive is the best option in this weightclass due to the greater forces that you mentioned, though it may be worth trying it to see (just stand well back incase it fails). As an alternative to timing pulleys, what about mounting the brushless motor vertically, attaching a wheel to it and running friction drive?

Don't think it's been done in that orientation before and doesn't call for as accurate a setup as traditional transmission methods; just have the motor mounted to a hinge that's sprung in such a way that it keeps a constant pressure on the disc (to account for wobble in the disc or wheel). You can then adjust the speed/ratio by varying the wheel size or how close to the disc's centre you put it.