Hi Roll'!
Welcome to the Thorn Cycling Forums!
First off, congratulations on your new frame. As a hobbyist builder myself, I know the thrill that comes with seeing your own dreams and vision brought to life with torch and welder.
As for your desire to convert a chain-drive Rohloff hub to belt-compatibility...I have looked myself out of curiosity and also found a dearth of information. You've done well to source the conversion parts. Lacking any formal directions, I would assume the basic procedure would be the same as replacing the chain-drive driver assembly and associated parts per usual, but using the belt-specific components.
Phil Woods manufactured a belt cog but not in production anymore and you still needed the carrier I believe.
http://www.philwood.com/products/bdrive/bdcogs.php
If my memory from a long-ago bike show serves correctly, Phil Wood made the carrier available with the belt cog; the carrier replaced the standard chain-drive cog, and the belt-cog simply bolted to it, making conversion a much more straightforward process. Unfortunately, they do indeed seem to be no longer available, judging by their absence from the Phil online store. Perhaps a query might result in the possibility of a one-off, though probably unneeded where you have the Rohloff OEM parts already on the way.
Thorn's long and successful experience designing their bikes specifically for Rohloff drivetrains makes them an authority on the hub, but Thorn have also decided against belt drive for a variety of reasons detailed in designer Andy Blance's "Living with a Rohloff" publication, see pages 22-23 of the latest edition, available here:
http://www.sjscycles.com/thornpdf/ThornLivingWithARohloff.pdfIf I were in your position, I think I'd review the hub assembly and service instructions and then view it as a parts swap. Another course might be to contact CycleMonkey (
http://www.cyclemonkey.com/ ) or Aaron's Bicycle Repair, Inc (
http://www.rideyourbike.com/ ) to see if they might offer advice, suggestions, or instructions. A photo on Aaron's site (
http://www.rideyourbike.com/internalgears.shtml ) does show a belt-drive Rohloff wheel boxed for shipping, so it seems they have some experience with them, and they do offer some non-standard services, like re-drilling Rohloffs for 48 spokes (a procedure adapted, I believe, from the original efforts pioneered by Andre's friend, Chalo Colina -- himself a remarkable machinist). Either shop might have more interest if you present yourself as the framebuilder who is seeking details on the procedure for your own OEM self-build.
Though this is not the specific help you sought, hopefully something in it will be helpful toward assembling the hub as you wish for your Dream Bike.
Best,
Dan.