can it be implemented in the game?
Well, infinite techs exist, and what I described for unit can be possible as I mentioned. Actually, I had tried to mod vanilla, and I managed to make a half finished Black's Mercenaries and a three-quarters finished Circle of Ossus; I have an idea of what is "easely" codeable (but I lack time. Maybe in July and August, I could try to remake them for WOL). I also doubt you can lock cards ingame. I don't know if you can set HCtechs to unobtainable, never tried. Also note that my ESO home cities are locked to a level of 131, so I can have max 131 unlockable cards; this civ will require many already unlocked cards.
Certain abilities are unlocked by tech: the sharpshooter attack of an explorer as most common example. The wol team knows a lot more about this than I.
Something that maybe also could fit this civ: you pay for a tech (infinite tech), that changes unit X to Y. When Y dies, 2 units Z spawn. (not 2 times X, else exploits could be used).
Maybe you should make a list with interesting ideas for the invisible weapons? People will tell you what is possible and what not.