This is 1) well known, and 2) from 2010.
It’s certainly an interesting design point but MicroBlaze is not known for compact code. It would be interesting to see it compared against something a bit more modern, such as RV32IC.
This style of instruction set has a long history in accumulator machines, especially in mini-computers such as the DEC PDP-8 or DG Nova and repeated in early microprocessors.
The big problem with it is it’s so tightly intermeshed with the implementation of the core that it’s incredibly difficult to do a successor with higher performance. It’s useful in a “simplest possible thing that can work” way, but not in a general way.
Doesn’t the Forth guy sell chips already?
Edit: Chuck Moore, and yes: http://www.greenarraychips.com/