I think the Iyonix is pricey (no more so than the Omega, I might add) is down to development costs offset against a relatively tiny number of sales (as has been discussed to death). I imagine the same applies to the Omega. I think the GeForce card in the Iyonix can be had for about £30 retail, so I would think Castle gets them a little cheaper, and they don't really represent much of the cost of an Iyonix.
With regard to ROL adding support for extra graphics cards, as no machine exists which can run Select, and also accept third party graphics cards (with the exception of VF, now discontinued, and of course, already working, no need for ROL to step in), I doubt this will happen any time soon.
I admire your optimism that MD will indeed ship an Xscale upgrade for the Omega, and maybe you have some inside knowledge that would give you cause to have such optimism, but I think you're on your own there.
Having said all that, if MD can pull something out of the bag and ship a fully functional computer with concrete advantages, good luck to them, but again, I don't think they will.