I've done a little digging, and have found about half of the story here...
When you import a file, you can select "all files" instead of just meshmixer documents and select a GCode file that way. It didn't used to work, but now it does. Yay!
And thanks to Chris, I've got an idea of the start and end code from the advanced slicer settings.
start_gcode = G1 Z5 F5000 ; lift nozzle
G1 E-20 F300
The start code doesn't have a G28 for homing or a G30 command for calibration though, so I suspect that part of what's needed is still missing.
Are we on the right track, Team FLUX?