What you did here with the greek units in the military barracks and the counter barracks is exactly how I envision an atlantean Poseidon. That would be so nice to see.
Theoretically it’s possible but requires a lot of replacements and triggers with conditions to enable units and techs in the correct ages. As it stands enabled units and techs are available for training/research immediately, there’s no option to make them appear but still be age locked.
Also, replacing the Mythic Age gods doesn’t work, since they require their own civs specific fort building. For example, the norse gods require Hill Forts.
The only other issue is that units and techs added to buildings they’re not supposed to be on don’t have hotkeys.