Positively love the shields, a great idea! Nicely executed too, with a clear heraldic style to the sculpt.
I don't know if this is (commercially) viable, but how about doing a sort of conversion packs with sets of separate torse+crests and matching shields? A buyer could then simply snip/saw off the existing crest on any figure (or flatten the top of the helmet for a crestless model) and add the crest and shield.