HHKB Pro2 uses NEC USB hub chip µPD720114, you can check its datasheet to know whether you can mod it. This is pic of Pro2 controller, not fine enough to read making on chips though.
Show Image
(http://i.imgur.com/gsLJ2Mf.jpg)
By the way, don't you have to use another terminal of DMM to measure current? :D
EDIT: Z8 is NEC hub controller and Z9 is national semi LM3526 power switch in pic above.
Doh! You are right. I originally meant just to test Voltage then thought I'd add in current as an after thought to see if maybe it wasn't supplying enough juice. I'll have to retest when I get home.
My idea is to take a small USB 3 port apart and strip it down to the PCB. Fit the female usb's into the USB slots, use the mini USB as the power source, Connect the USB port to the female ports out, probably with a ribbon cable, and connect the HHKB in more or less the same way to another usb port. If I could preserve the original build as much as possible that would be excellent.
Due to possible space constraints a teensie might be needed.
I bet that 100ohm resistor is our culprit. That resistor is probably protecting some delicate ICs though.