Actually, I asked the guy in Thailand about making a version for the UC300ETH.
He said it's unlikely, because they made a Pokeys breakout board and he's not selling any.
I think it's partly because:
1) Nobody has heard of CNC Room.
2) There aren't that many Pokeys users.

I think CNC Drive, like CNC Room, are afraid of spending all the time developing a quality board, and finding out that they don't sell any.
But if you build a quality product, and let people know about it, you'll sell a lot of them.

There is a minor issue with using the MB2 with the UC300ETH.
The MB2 is designed to power the Smoothstepper through pin 26 of the ports.
But the UC300 also supplies 5V power through pin 26.
I see two option.
1) Disable the 5V power supply on the MB2 by removing the solder bridge.
2) Use 25 pin cables.

I plan on doing both. I'll make an interface board to mount the UC300 to the MB2, and make some custom ribbon cables.
It won't be quite as clean as the ESS, though, because the UC300 ports are different.
Instead of plugging in ports 1,2, and 3, they need to go 2,3, and 1, so the cable will need to be longer,m and cross over themselves a bit. I'll know a bit more when I get it.
But it looks like a really nice board, with all the features I was looking for.