Something interesting I noticed on the JP ordering page. Where it says "Platform" you can choose between "Normal edition" and "With Prime Day commemorative bonus". If you choose the normal edition you can only order the CoreGrafx or TurboGrafx-16, but if you choose the Prime Day version you can order all 3.
I'm not really sure what to take from that, and I doubt there is any actual "bonus" for ordering the Prime Day one. One possibility though is that they are only allowing us to order the white one on Prime Day and after that only the CG and TG16 will be available outside Japan. This could be the "Special Edition" thing they mentioned in the tweet about making all 3 available in each region in limited form.
In any case, I've decided to stick a pre-order in for the white one (there's only a couple hours left as of now) just in case it turns out as some sort of rare limited release opportunity. I'm fully expecting to cancel the pre-order at some point in the next 9 months but maybe something cool will be announced (like M2 doing the emulation).