[Nige]

Forget the USB port
Buy the Harley accessory pig tail, cut the grey 6pin female connector, you'll be left with 3 wires, red (battery 12V, violet (accessory 12v), black ground.
Find a MX150 molex male connector (3 pin, weatherproof) available at digikey for about 5$ shipping included)
ground pin goes in the middle, violet goes to the right or pin 1 when the tip of the connector is on top looked at from the wire side
hook the connector to the P&A port under the seat, right under the plastic box between shock and RESS.
et voila, 13V clean, 15amps capacity to hook your phone charger or heated vest, etc (max 15amps tho)

Is "P&A" short for Power & Accessories? Where is this port getting its power from - direct from the Dc to DC?

 

[Nige]

Exactly. It is hooked up to the RESS to 12V converter. Same input as the 12V battery charger.
Works marvels for my USB phone charger and heated vest.

Do you mean to this location?

 

[Nige]

this component appears to be the "inertial sensor", made by bosch. I think this is the generic part with plug and pin out details

No. It's attached to the bottom of that black plastic cover. It's awkward to get to… but I have connected into it via a Molex 150 3 pin connector. It is totally different now I have real 12 volts -15 amp supply. I can post some pictures to show you what I've done?

Photos would be awesome- also part numbers for the Harley Splitter cable and the Molex plug if that's not asking too much

 

[Nige]

So, I'm still waiting on the Molex connector to arrive in Australia (!) but did find a dual USB outlet that is perfect, so everything is ready. Double sided tape to the headlight frame and the standard bale is the perfect length to reach the enclosure, where it will plug in to the fuses adaptor and Molex plug. Added some cable sheath for good measure. Link to the socket is here Dual USB Port Motorcycle Fast Charger Socket LED Voltmeter Display ON OFF Switch | eBay