Connect USB to microprocessor lacking a output port?
There is no guarantee that the manufacturer put any way to program the MCU onto the PCB. 

MCU's could be programmed before assembly.

It may be infeasible to program the MCU even if you could get at the right pins. Most MCU's have built-in mechanisms to allow a user (in this case camera manufacturer) to prevent a chip being reprogrammed, or the program read. It is likely that the camera manufacturer used those mechanisms.

It is sometimes possible to defeat those mechanisms, but those forms of 'attack' may damage the chip, and make it unusable.