Oops, it looks like I got the order wrong there.
|"master" /dev/something virtually --->
||/dev/ttyAMA0 serially over USB -->
||USB Type-B port
In theory, I gave you the link before for virtual serial.
This page might help to understand a little better what's going on. Hopefully, I'm not sending you off on a wild goose chase.
I'm pretty sure that OctoPrint only includes a short list of possibilities in that drop-down list. So you'd need to adjust the Settings to include trying other options like these exotic device names.
Stepping back from all this, I'm not 100% convinced that a second computer is required. What if there is some service that runs on the Raspi which presents the new pseudoterminal to the printer but has OctoPrint attach to the
/dev/ptm0 perhaps? In fact, if this works any number of man-in-the-middle activities could happen to include... job resumption upon loss of power.
I'll eventually put some time into this; just not today.
Normally, I would just write a Go language CLI app to do post-processing of the GCODE file, programmatically swapping the Y/Z for something like this. And then I'd send the modified GCODE to the printer. Honestly, command line goodies like AWK could do this if you're into terse UNIX commands.