Hi,
I'm not sure if i understand you correctly.
So basically you would like to have the 'planarface' of a rectangular column, right?
You should be aware that columns are type of family instance, so the geometry is nested one level deeper.
E.g. Walls and floors are not family instances, they have their geometry directly in the first level of the geometry hierachy.
Use the RevitLookup and explore the structure between those types of elements.
So different type of element, different code to approach.
Looking at the Revit API Developer Guide code example (GetAndTransformSolidInfo) you could use the GeometryInstance Class to use the GetSymbolGeometry() Method to obtain the solids from a rectangular column.
Not sure if this is what you are looking for, anyway good luck!