Is a simple but important question,
CardMaker can replace an empty plane exported from modeler?.
The only way “planes” appear in Egg files is as infinite planes used for collision geometry. So no, because CardMaker produces regular, not collision geometry, and what it produces are finite rectangles rather than infinite planes.
I think by “plane” the OP really meant an ordinary polygon.
So, yes, a CardMaker can certainly replace a polygon exported from a modeler.
David