The device I'm connecting to the Polaris is one of those $10 made-in-China USB MIDI interfaces, so I'm sure strict adherence to the spec wasn't their top priority. ;^)

Also I made an assumption that I probably should not have made... they may well be using pin 2 as the return and not the shell.  I think I just read somewhere that some interfaces use the shell as a ground/return so I assumed that's what this device was doing.

According to the Polaris schematic, pin 2 and the shell of the MIDI output connector are both connected to chassis ground.  I'm not too keen on connecting the shell to digital ground, but connecting pin 2 might not be so bad?  If it turns out the interface is using pin 2 as a return maybe I'll try that.