Comment 11 for bug 895223

I admit to not reading the follow-up comments just yet, but I'm wondering if a very simple version of this request couldn't be implemented simply by allowing "phantom building" of sorts.

Basically, you'd have "defense" buildings as usual, BuildLimit=1 or even -1, but if you clicked on them when they're ready, you wouldn't get to build them, they'd just quietly get added to the list of structures you possess in the background, without appearing on the map.

That way, the normal Prerequisite logics should apply, and all we'd have to do is ensure that the technologies are Insignificant without causing BuildLimit glitches.

They would still need ConYards to be built, but unless D knows of a reason that speaks against phantom buildings, I figure this could be the easiest way we can implement non-tangible prerequisites quickly.
Depending on the way the game is coded, it could even have the advantage that you could use standard building effects like InfantryGainSelfHeal to research "healing nanobots" or whatever.

It would by far not be as flexible/complex as other stuff proposed above, but I do think it'd suffice for the majority of users, and be easy for us to implement.