I have contacted Sandy, the Master, about this, but whilst waiting for his reply, I thought I'd bounce it around here.
IBM Japan's equivalent of the PCjr was the IBM JX, which was also sold in Australia and New Zealand. The JX was actually a relatively successful design, and one of the advantages it had over the PCjr has a decent keyboard -






As you can see, there were two versions of the keyboard - a compact one that was IR only, and a fullsize one that was either IR or detachable cable. From the pic with the removed keycap, it looks vaguely like a white Alps (bare in mind that this was a good few years before the white Alps that we know went into production) and I'd bet that the half height keys are the same half-height green slider switches that were in the PC Convertible keyboard that someone took apart a few weeks ago.
So, anyone know anything? =P