I suspect the STLink V2.1 would fit in a C8, especially as most C8's are really CB's
But I think the point is that the STLink (even V2) is not intended as a bootloader its a debugger, and would severely limit the amount of available code space left for applications.
The BMP is pretty much the same as the STLink, except without mass storage, and the BMP is quite large as well (i.e it would not be useful as a bootloader)
I think a mass storage bootloader would be great to have, but as using the HAL etc or any other lib e.g. libopencm3 would be impractical due to size, I doubt its practical