I just have an extra-long battery tender lead set attached to my battery, and I coil the length of cable up and tuck it into the toolbox.  On long trips, I unwind the  cable and route it out from under the seat and up to my tankbag.  I have a powerlet socket adapter in my tankbag - it was intended for someone to drill a hole through their tankbag and mount it to the bag's side, but I just leave the thing inside the bag.  I plug the battery tender cable into it and I then can plug in a cigarette-style power connector/cell phone charger in the other side of the adatper, or with another short adapter cable, a GPS (If I had one) there.  I cannot recall the vendor at the BMW rally I got this stuff from, but they gotta be advertising in either the MOA or RA magazine somewhere.... will look later this week.