SDK or Python would be easier / more sensible.
You don't really need new tags - all the information is already there, stored as the units pre-req tech.
I dont know the exact location of my head, but i would expect it to be in CvUnit.cpp.
Another thing to concider would be - how often do you happen to get a unit without the needed tech ? The only way i can think of is gifted units - which does not happen a lot...