After doing some Googling, I discovered that there was a company called Microdata Corporation: (https://en.wikipedia.org/wiki/Microdata_Corporation)

They apparently existed from 1969 to around late 1970ish or perhaps early 1980ish, during which time they made computers which were open to Microcode programming by the end user -- they even gave the end user a manual on how to do it:

http://bitsavers.trailing-edge.com/pdf/microdata/3200/761020...

This manual should be entitled "How to build a late 70's style Microcode programmable computer". It really gets into the guts.