I recently discovered that units using faction shields are not actually limited to the pre-defined ones in the unit_prod files (like the feudal men at arms and chivalric knights). It is possible to create new units that use the same FACshield without having to define much else.

For example, I have created what is effectively a 'feudal axeman' unit, by adding an axe to the animation BIF, then defining it to have the same shield as the FMAA (from the same folder, obviously). When it appears in battle, I was just expecting the blank shield to show without the heraldic overlay, but no, the overlay does appear. All you need to do to get the overlay is simply use the same shield as the existing unit. This means there are loads of possibilities for new units all bearing faction heraldry. Just re-use the shields that have the overlay.