If the script is usb_power aware, you could have more complicated signalling, but I'm not totally clear how this would interact with sync.
Hmmmm ... would not be hard to extend the USB remote code to "recognize" some special pulse sequence unlikely to be generated by a standard switch unit. ( e.g. 50msec on/off three times). That would let you switch to playback mode and you could use the playback mode = PTP option. Or just drop USB remote enable.
I suppose the code could also watch for a complementary "enable" pulse sequence. (e.g. 50msec on/off five time).
Although that gives me nightmares about trying to figure out how the original code tried to recognize that a Ricoh CA-1 was connected.
Can you do a synced shot that is imitated by shoot() in script?
I think so - as long as you have the remote enabled & sync enabled. Set the Control Mode to [ None ]. Then just fire the shooting sequence as soon as you see the 5V. If sync is enable, it will wait for the 5V to go away before completing the shot.
Another possible issue is that play / rec mode switching can interact weirdly with PTP. If you switch from PTP and then unplug the cable and plug it back in the camera can get into weird states.
Does the PC or camera get confused at that point?