The rear speaker connectors shown on the wiring diagram don't make sense since there is no provision for passenger control functions, e.g. PTT, volume, etc. that come with the HD rear speaker pods/speakers, and the amp is not wired to the HK head unit to accommodate those functions. So in terms of the "rear speaker" connectors from the amp, HD doesn't seem to offer a product that would work with those connectors.
In a non-SESG2 installation of the speaker kit, you'd use the 70169-06 non-Ultra overlay harness to provide the connection to the 35 wire connector on the back of the HK radio to access the rear speaker pinouts, passenger controls, etc. The SESG2 already has a 35-wire connector installed but only uses 4 or 5 of the pinouts for the ipod interface. Perhaps the "no fit" for the SESG2 is due to the ipod connection already being there at the back of the HD radio.
Having said that, however, it's clear that Ultras without the ipod interface can add the ipod interface by adding the wires to the pre-existing 35 wire connector. Don't see any reason why you couldn't do the same thing to an SESG2 by adding the wires necessary for the rear speaker kit, and leave the amp wiring alone. Probably time to call Harley Tech Services.
Carl
PS: No, I don't know for sure.