I have been thinking for a while that Chibios could be a nice option for the Quad. It already has an USB driver for the STM32F1, and because we have the 256 kB of extra flash waiting for us, the size is not a concern either.
In fact the situation for software development on the Quad seems now better than ever. There is plenty of examples and starting points, the flash size situation has been solved, FPGA can be programmed, and there is even the competition with cash prizes. Only thing holding us back is the number of hours in a day 