In feudal and early castle age, giving extra armor as a civ bonus is equivalent to make pavise a civ bonus. Equivalent in the sense that it is the same. You will have xbow with +1/1 armor.
For instance, if they get a new UT very weak (like the hun one), their late game will be unchanged, since your units already have the extra armor as a civ bonus.
Then finding a new UT more useful is not that difficult imo. For instance a UT giving a discount to condos (even for the all team), a UT affecting monks, rams, buildings. All these options can have a limited or situational effect, also negligible, so no balance issues at all.
I think this would make more flexible the way to balance them. For instance assume someone argues that +1 PA stackable with the current pavise is too much since you will get +2PA arbs in imp (btw, I am very convinced it is not). Then if pavise becomes a civ bonus, nothing changes since you will still get +1PA arbs in imp.