"Unit construction only" Space Yards

[dmm]

There is already a "unit construction only" facility. It is called a planet.

Sarcasm aside, though, this means that there is a flag somewhere in the code for "unit construction only." MM just needs to make it available to modders. But as you point out, the AI would need serious reworking to make proper use of this, which is probably why it has not been made available.