For example a pod underneath a skycrane, or a big tank on a trailer

Implemented in V18.4.0.

Do-able, but would require some work to differentiate between parts with those hitboxes and those without. You'd essentially have to define them like you do vehicles. I'll add this as a possible to the list.

This would be great to have for things like different landing gear configurations or folded wings as parts on planes as well.