Low voltage means high current, high current means thick wires, thick wires means...
If you could build a 10 ounce plane you'd need 70W on average to fly it with a brushless. At six volts, that works out to over ten amps, which is going to call for at least 26 AWG lines -- and that's just for lines that don't melt, not lines that don't drop more voltage in the lines than in the motor.
I think we're stuck with ESCs and LiPos -- besides, a cheap brushless motor plus a cheap ESC works way better than a cheap brushed motor. I don't even know if Astro Flight or anyone else still makes the super-zoot brushed motors with rare-earth magnets.