STM32 firmware upgrade (In-System Programming) over USB?
It is OK for prototyping phase to use aforementioned methods (1-4), but it is unacceptable in a real production device.


Are they really? I know many products that use any of the above simplified by a simple gui tool. If they don't have WiFi ota updates... Except the SD card one, everyone knows how to use that.
Are you expecting users to update very few days? Since if you are, I strongly suggest writing better code.

Anyway, the USB MSC DFU in the LPC11U24 just writes a bin file directly to flash if the checksum of irq table is correct. Doing the same for hex files would be challenging.
The bootloader also cannot recover from failed writes, since there was never room to cache the entire binary. It's really just a gimmick, and they haven't really put it in any other parts since these few select parts.

It is up to you if you want to add a little flash chip you can perform the USB MSC on, and the reboot loading the new flash image from that. It should be perfectly possible to do this reliably.