...I was thinking of the mechanic being rather like a "troop ship" - when those 5 (or however many) troops deploy, the ship is GONE. The "production module" would be something like that, only moreso: in the process of building a planetless Outpost, the starship itself is cannibalized, incorporated into the finished structure...
So, what you're suggesting is actually something like a "base", as in "large, immobile structure floating in space". This is a concept that is missing in FO until now, but might be an interesting addition (I don't count the immobile hull types "Colony Base" and "Outpost Base" as implementations of that concept). It would allow for things like "space stations" and "starbases" et al.
Introducing this kind of immobile objects that can be build by players of course raises the question how the construction process of these things shall be designed. The simplest approach would be like ships: they can only be build in systems with a spaceyard. This of course would limit what kind of things can be done with these "bases" - it wouldn't be possible to do the things you suggested for example.
Another approach would be what you described: Provide some kind of ship that basically acts as "constructor". This "constructor" ship can be build like any other ship at a spaceyard, moves to the location where the new base should be placed and proceeds with "constructing" the base, which can be done in several ways. In the case you described it would mean to decide what kind of base the final structure should be by selecting the desired building, investing the required PPs, and the "constructer" ship will be consumed in th process.
Designing "bases" this way opens up far more possibilities what can be done with them. I've to admit, I like the idea very much, maybe we should open up a new brainstorming thread for that to discuss some details? What do you think?