Much to my annoyance, I could find no single mod that upgraded Boston Airport, Home Plate, and the Mechanist's Lair from partial settlements to full-fledged settlements - and of the mods that did touch one or more of these workshops, many seemed to lack part of the functionality required to make them function as normal settlements. Several hours of reading and xEdit later, I came up with my own attempt - with this mod, it should be possible to assign supply lines and send dismissed followers to these three (no-longer-)pseudo-settlements, with full access to the building options that other settlements have.

Requires the Automatron DLC. That's it.

I think I've covered all my bases, but feel to let me know if I'm missing anything that would ensure complete functionality.