[DanHoward]

Originally Posted by CarrionPeacock

I believe the 'coat of plates' refers to the vest being worn here. It seems to be a sort of early brigandine over mail.
However what I had in mind is what's shown here and it is early breastplates, vambraces and poleyns over mail. According to LowTech Load-outs, the official way of doing this is layering light mail armor with medium plate armor, stacking their DR and causing DX penalty.
My idea was to instead use the mail-and-plate armor to avoids DX penalty and use the Heavy Plate rules to make it stronger. They're designed and built to be worn together so it shouldn't be as awkward to move in it as any other random combination. The resulting armor is much heavier than layering mail and plate so it's not particularly munchkiny: Light mail and medium plate gives DR 9 and wieghts 32 lbs, mail-and-plate with DR increased to 9 using the Heavy Plate rule would weight 60 lbs.

It won't really break anything but it isn't a practical solution. Once the plate gets to a certain thickness in the mail-and-plates construction, it stops flexing/articulating properly. So you get DX penalties anyway.

The best way to address the problem is to use mail gussets/voiders like Sean suggested.

Keep in mind that 14th century plate over mail combinations were primarily intended to be worn by cavalry. The weight burden and movement hindrance isn't so much of a problem when in the saddle.