This recent thread about microprogramming VAXen was interesting. If you
read Dave Patterson's history of VLSI RISC machines you'll see that he was
interested in programming the 780 but changed his mind.
It's also interesting to note that microprogramming was used in two
different cases to create capability machines: one, a 11/730 at Cambridge
and the other the 11/40E at CMU.
There's the KMC-11, but that's another kettle of squid.