Interesting idea, actually. I have never thought shield skill as a requirement to use a shield was a mechanic conducive to balance. I don't think I agree with a 36/3 build being able to use huscarl shields and steel shields. Maybe something along the lines of
Shield Requirement = agi & str >= shield skill req*4
So a 21/18 build could use a 4 shield skill shield but a 24/15 build could only use a 3 shield skill shield.
Alternatively, remove requirements altogether but for every shield skill level that you lack for a shield you have equipped, reduce the shield's loom level by 1, to a minimum of -4. So you may be able to equip a huscarl or a plate shield, but without sufficient shield skill, you'd most likely be better off using something else.