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rjhammett That's a neat idea. Looks like a modified front reflector mount. It puts the cage a little lower than other handlebar mounts; i wonder if you could flip the mount around to put the bottle higher for easiear access...I had a standard TA bottle cage mounted one of those handlebar adapters. I found the weight on the forward end of the cage started to bend downward. After losing a bottle under my front wheel on a bumpy section of trail, I took to tying one of my wife's thin hairbands around the top back of the cage and looped it over the nozzle of the bottle for extra safety. After several months of slack, bend back, slack, bend back, the alloy TA cage eventually snapped.
Note also that the vintage handlebar-mounted TA cages were designed differently than most traditional cages designed for down and seat tubes, having no openings on the sides and a tall loop extension at the front of the cage. How's your's holding up? For a setup like yours, I recommend sturdy cages like the King cage shown above or other chromed/stainless steel cages. (Love the Holdsworth, too.)
If you look closely at the photo below, you can see the stretchy hair band and the cracked cage (bottom right):
