I don’t see why not, but there are some major problems with it:
- F4 for searching polygons inside polygons uses DB function, JOSM don’t have any fast and indexed function for that, so it have to be implemented.
- Searching entire data set for polygons inside polygons will be slow.
- Personally I think it is an error if building relation don’t exist. I see checking for that more as validation rule.
I will think about it in future but don’t expect it fast…