I think Wraith built a Mothership for his HomeWorld mod, it was beautiful!!!! But i think he had the same difficulty, it couldnt actually build ships like the one in the real HW.
But it was an intersting idea.
And the spawnable ships are possible, check out the abilitydeploycombatturret and the frigatecombatturret. You can also change the 180 second life span of the ship to -1 so its permanent. So we can get ships to spawn other frigates, just not caps yet, though, the frigate caps is possible. Ive seen that done in some mods. As for a ship building real caps, wont the starbase do that in Entrenchment? If so then it just a matter of getting the starbase spawnship in the fleet out to the other planets.
I dont know about vanilla sins though, 115. I dont think i saw it done earlier, and 115 i dont know, but it may share certain things with Entrenchment, and if it does it, but im not sure on that, it would be way cool, and get you almost what you are asking. 
-Teal