In interest of progress, posting an update on this topic.
Calfee has finally announced the availability of TA rear dropouts (see post
#2 above). 2014, it's about time!
As posted on Facebook:
6/4 titanium, 142 thru, available option on all models; road, adventure, CX, single and tandem.
Had this been available a year ago, I would have used it along with 135mm spacing on our tandem built last spring. The availablity of dozens of X-12 142mm hubs (135mm frame spacing) is very appealing. A few months back, I discussed with Calfee the possibility of modding our existing 145mm frame, but that would require a major rework and not feasible. Apparently even pulling an existing dropout is also a very big deal, so swapping to this format may only be realistic for new builds. Maybe next time.