The Tofu kit uses a hotswap PCB and if you look at the pic there's a big left shift with no place for the extra ISO \| key, wouldn't be surprised if there's no space for ISO enter either. Even if there were spaces in the PCB as it's hotswap the switches will not be held in like they would be when soldered so making extra holes in the plate/case is not a good idea (at best they are unlikely to be straight, worst case they will fall out)
One of the main features of the HHKB layout is that backspace is easier to press - making an ISO version would lose this. What about the HHKB is it that you like?