The shields in the ruler designer are those used for the creation of random dynasties. E.g., when you create a baron out of hammerspace, he is given one of those randomisedly atrocious arms. The GoT mod adds a few features -- replaces the crosses with seven-pointed stars, for example -- but doesn't touch arms creation otherwise, which makes sense since many armorial bearings in the source material are atrocious abominations. The arms of the named, pre-existing houses -- Targaryens, Starks, Hollards etc. -- are not created using the ruler designer's elements but custom, handmade files in the gfx/coats_of_arms folder. This works the same way as the Dynasty CoA DLCs for the base game. That is because the random arms designer and its ruler designer version are severely limited in what they can do, and in how they can be modded.
What can be done is replace the elements and colours present in the ruler designer for more aesthetic random CoAs, as has been done by the Prince and the Thane.
