I've seen some contemporary illustrations where people are depicted wearing brigandines with what looks like a plackart, a piece of metal armour covering the non-ribbed part of the torso. I cannot for the life of mine find out whether this piece of armour worn by itself is also called a plackart, nor can I find out how this piece of armour (or the plackart) is called in my native German - be it now or in medieval times.
The whole assembly consisting of the said element protecting the abdomen, hips and thighs are often referred as Magenblech in German, especially in reference to this particular element protecting the abdomen and reaching up to the sternum, while the lower parts of the assembly, protecting the lower abdomen and thighs being called Bauchreifen (sometimes an older term Leibreifen is used) or by more modern designation Bauchschurz, sometimes consisting of several articulated parts. The term Magenblech is also used to describe the lower part of a breastplate or cuirass, reinforcing the abdomen area.
Please note that in the contemporary continental depictions, such piece of armour is rarely worn alone and most if not all historical artifacts from German-speaking areas have holes for rivets or screws, strongly suggesting that these were most commonly meant to be attached to the upper part of the breastplate rather than worn on their own.
Boeheim, W., Handbuch der Waffenkunde: Das Waffenwesen in seiner historischen Entwicklung. Fourier Verlag, Wiesbaden 1985.
Wozel, H., Turniere. Exponate aus dem Historischen Museum zu Dresden, Berlin 1979.