Hi Degsey,
BMW do a simple (and cheap) one that fits in place of the plastic cover over the captive nuts for the handlebar height adjustment. It puts the sat-nav between the two bars.
I have one fitted to my K1300GT and it's an easy option - although I prefer the sat nav mounted a bit higher - ie above the speedo as it is on my R1150GS.
I can send a pic if thats any help?
Cheers.....................Grizzly
OOPS!.................MY TYPING IS WAY TOO SLOW!!!
Grizzly's sounds like the one I have on my K and the spare I'm selling. It has the c.2" square of 4 holes that most kit mounts to, including my ancient NavIII.
I use a Garmin Zumo 350 and had to drill extra holes in the BMW bracket so the Garmin powered mount would sit centrally on the bike - no real hardship though - I think you could fit any sat nav mount to the BMW bracket fairly easily
For power, I used the Canbus socket that's meant for sat nav - it's stashed away to the front/right of the battery with a blanking plug in the socket - but you need the special plug and cable tails from BMW etc.
There's a 12V socket by the gear lever but I believe they refer to the spare lead by the headstock as an accessory socket; you'll need the right connector to use it. I'll do more digging to confirm my recollection!
BTW more images of the bracket here. (Slant K1200GT is same as K1300GT).