I have good and bad news. The bad news is that there is no easy fix for the current native heraldic armors. The good news is that it's indeed possible to have heraldic symbols anywhere on an armor without seams.
The reason for the seams are twofold. Firstly, there is a cloth structure baked into the banners themselves. This cloth structure is also applied to the armor, so that when the symbol is positioned in the native position, it fits right in and there are no seams (at least not due to this cloth structure). However, when the banner is moved (e.g. shifted up to display the symbol on the chest), the cloth structure no longer lines up, and the edges of the banner become visible.
Secondly, and this is the much bigger cause for the seams (the reason why there are seams even in the native positioning on e.g. the left side of the heraldic mail with surcoat), the banner mesh seems to be lit with a point light or something. This causes the symbol area to be lit more brightly than the edges of the mesh, and those darker edges can then be seen against the lighter background of the rest of the armor, if the banner is moved around. With the help of cmp, I have managed to make the banner mesh uniformly lit by inserting this into the respective armor's tableau definition (right before the end):
(cur_tableau_set_ambient_light, 999, 999, 999),
Now, if we both remove the cloth structure from the banner and apply this lighting fix, we can make a nice heraldic tabard:
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loginBut as I said in the beginning, this doesn't work like that for the native heraldic armors, because they have a very blurry and undetailed base texture, which becomes very obvious if you remove the cloth structure from the banner:
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loginSo, what we need to make those armors look good is someone like Zimke to paint some cloth structure into the base texture, then we can apply above described technique to make them look nice.