What about pushing tempered chocolate through a ricer or sieve, and cutting it with knife when the drippings are about rice sized.
I'd say you could drop it into cold water to solidify it instantly into the proper sized pieces, but that would of course mess up the temper.
If you could figure out how the pasta Orzo are made, if there's a mold for that, could be adaptable to making white chocolate rice. If none of these work, might be worth getting a vacuform setup for a few hundred $ and just using rice grains as the positive for the mold.
I'm in Thailand now and just met the King's palace manager in Isan whose daughters are in Auckland studying right next to you! I should send them over to see you. We're going to try to help them grow good cacao varieties up here, so look for some tasty Thai chocolate (with lemongrass) in the next few years!
Nat Bletter, PhD
Sounds like you are having a good time in Thailand!
Hadn't thought of pushing through a large sieve, that could work if we hang the sieve up high & leave the strands hanging until they have solidified. Then take off and cut to size.
As you say, the other option is to get a mold made from actual rice, which is probably the best option long term.
In the long run, the best bet may be - depending on how much you want to make - to buy a chocolate "lentil" machine (think M&Ms) and get a new set of molds made for that. This will form full 3D grains, not half-grains as would be the case with a conventional mold.