Make a sketch an the desired plane. Use "Convert Entities" to project
the boundary to this sketch. Finaly make a Planar Surface from the
sketch, and this surface will have the projected area.
Or at least: this is how I understood your question.
Did you try to make 'silhouette' split lines so you can create an edge
on the model? Those edges ought to be able to convert?
Intersection curve might also be an option if the widest extent is
planar or on an easy to define surface.
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