Its also probably wise for all designers to start taking HHKB/660c boards into consideration as well with the MX Compatible sliders right around the corner. Could add them to the non-standard adapter kit.
I know this is totally off topic but do you know if there are SA spacebars in 6u that would fit the HHKB/Topre boards with the replacement sliders?
DCS has a 6x size spacebar possible, but I don't know about the stem positions. For the other profiles:
If I had a HHKB with MX sliders, I'd probably use a standard 6.25x spacebar and 1.25x caps on the mods next to it. Leaves small gaps to the 1x keys, but should fit at least. If you grind the case you can fit 1.25x caps on the outside caps, too. No gaps and standard caps will fit.
The FC660C can be done with 1x, 1.5x, 1x, 6.25x, 1x, 1.5x, 1x. Leaves a small gap on the outside ends of the row, but no gaps between caps.
Realforce can use: 1.25x, 1.25x, 1.25x, 6.25x, 1.25x, 1x, 1x, 1.5x. Leaves a gap at the left end and between the right hand 1.25x and 1x.. Could use 1.5x, 1x, 1.25 on the left side to make the gaps consistent instead if you prefer.
So all three boards can be "supported" with standard base and Winkeyless kits, assuming the stems are in the right places. Only other options are:
1. get SP to make a 6x spacebar for the other profiles.
2. use stock spacebar on stock slider.