I don't think you would want to use a specific 'beam' or 'truss' object because of the fact that it does some simple math. Then also the question is who uses that object --the structural engineer will want a program that calculates the whole structure, and typically will not be interested in 'modeling' beyond structural diagrams; the architect will be more interested in the shape and dimensions of the pieces, and how they get joined to the other stuff; the builder couldn't care less about loads.
I think a potential market for stuff somehow related to your idea is system builders, and component manufacturers. Say, a stretchy pre-engineered building, or building components (metal frames and decks, precast concrete columns beams and slabs, in isolation or working together, wood trusses), that take the right dimensions depending on load, span, height, etc. And there the issue is that the ArchiCAD user base is not huge (I guess most manufacturers don't know of its existence at all) so you need to make a very good case to get the guy interested in investing in object development.
