Success!
I was able to get all but one of the Hepco & Becker rack's mounting points to line up properly without stressing or bending any metal. The right-top-front connection required an
adapter I fashioned from some scrap metal. The holes there were about one inch off. Additionally, even without the horizontal connecting bar across the back of the racks, the assembly is rock solid.
I ordered the smallish
Happy-Trails Owyhee tragkorbs which are just the right size to fit the K75 without creating any obstructions. Their 14" height allows both tragkorbs to be mounted evenly without the bottom touching the muffler on the left, and without the top interfering with the hinged open seat on the right side. The 7.5" width means the tragkorbs stick out just about the same as the BMW system cases do and they offer about the same capacity.
I used Touratech mounting hardware which is designed to fit the 15mm H&B pipe width. Measuring and drilling the holes in the tragkorbs for the mounts was time consuming. I started by supporting the left tragkorb on wooden blocks and wedges to position it barely above the muffler, and angling it so that it's matching counterpart on the right side would clear the open seat, and the rear tragkorb latches would clear the turn signals. It's a more drastic angle than ideal, but it's the only way both tragkorbs will fit without creating problems. Each tragkorb mount was individually held in position on the supported left tragkorb, traced, and then attached to the tragkorb on a workbench. This process took a while but assured that the mounts fit perfectly and would provide the most structural strength and eliminate tragkorb slippage once mounted on the rack.
I then removed the mounts from the left tragkorb, placed the left tragkorb on top of the right tragkorb, and traced the holes onto the right tragkorb with a silver Sharpie. Attached all eight mounts to the tragkorbs and then both tragkorbs to the rack. Vacuumed all the aluminum shaving from the tragkorbs. Done.