The background is here

https://www.eevblog.com/forum/microcontrollers/32f417-usb-fs-(not-hs)-and-dma/msg3958874/#msg3958874

and here

https://community.st.com/s/question/0D53W00001KTXLFSA5/how-to-integrate-slowwrite-spi-flash-into-stm32-usb-device-library-to-return-ok-csw-when-flash-write-finishes

but nobody seems to know.

However lots of people must have gone up this path...

I am happy to give somebody some money to do this bit of code.