The Plug-In should recognize which parts of the model are technically removable and check them for size. Smaller parts could then be removed, maybe using a percentage slider or a slider relating to the size of the objects to allow some kind of control of the amount of removed parts. Feasible?

Almost feasible.

Unfortunately there is no "DeleteObject" function available.
However, objects can be cleared so they have no points or faces, but remain in the hierarchy.

A bit of 3D pythagoras on the object bounding box will get an approx size.

You just need to scan all objects for all materials in a group first and which objects have those materials.
Then remove objects below a certain size, making sure it is not the last object in the group with that material.
Not a very simple job because slight variations in the lighting factors can produce the same material in MSTS but are different materials in 3DC.

Almost describes it best.
I think there must exist a way to leave out entire groups. Kuju tracks have a group named railtops which is not existent in the faremost LOD. It would be nice if that could be achieved with 3DC.