Projected Surface Area

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.

Thomas

I already thought of this as a work-around solution, but I was hoping there was just an option in the measure tool I didn't see

Ewout

This doesn't work because of the complexity of the shape by the way (it won't do convert entities, not even when I select the outer edges manually)...

Ewout

A bit of a reach, but...

Try using a "Ruled Surface" to extend the face edges in a specific direction. Then cut the ruled surface with a plane and use those edges.

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.