So, I tried using the "unit production" on a meta-unit to see what it would do. I queued a few units into my meta-unit but it didn't seem to have any affect.
I'm guessing what that does it add it to the production of a nearby factory or assembly plant?
For me, personally, I'd prefer if this worked slightly differently.
It would be nice if, when selecting a meta-unit, you had all available, non-grouped units, displayed on the right panel. All numbers of available units would be displayed above their icon and, if you clicked on add one, it would take the closest free unit and add it to your meta-unit. Also, one could remove a unit a similar way. That way, you could break off units from the meta-unit in an easy way. In this way, it would be easy to adjust the numbers in each meta-unit.