Here is what my local dealer and I came up with as a solution to mount the Denali D2D's to my 2013 F800 GS. I was struggling with a location to mount them until I saw this thread. (thanks jengel451 for the link to a part number!)
I didn't like the mount from TT because of the location next to the headlights and in front of the turn signals. Not a big fan of having them on the forks, had tried another location and was about to put them on the outside of the fender when Charlie came along...
This set-up consists of the BMW bracket that Charlie refers to, the hinge mount that comes in the box with the Denali's and Twisted Throttles M6 Hinge mount (ordered separately). They are then placed together in an "H" pattern with a few extra washers to make it all fit snugly. So far it is working really well for me: rock solid with no bouncing lights at night and very easy to fine tune the alignment of the lights for aiming. Eventually I might just drill through the factory mount and remove one of the brackets to tidy it up, but I am going to ride this way for the time being.
Denali D2D lights from TT: $360
OEM Bracket from BMW: $135
TT M6 Hinge mount: $5
Labor and shop materials: $95
Total = $595
Still expensive, but it sounds like it is less than BMW's option and I have a high and low beam. Needless to say, I am a big believer in seeing and being seen.