It probably still needs more tidy up, but I uploaded the code to Github.
At the moment, it's expected to be uploaded to the Bluepill by the dapboot bootloader, so the "make" output won't work out of the box.
The steps would be to flash a FW image of the dapboot bootloader to the Bluepill with the ST tools , and the use the DFU tools to flash the application.
It sounds unnecessary complicated for a development stage, but it's a better solution in the long term, as it will allow normal, non technical users to upgrade the FW.
Hopefully will be able to upload more instructions and tidy up some things in the next couple of days