Adding a specific check for `of` filters would just be silly.  If we don't want to allow it we should do so on code review not as a hardcoded check.  Especially given the fact that aliases allow them to get around any hardcoded check we provide.  There is no reason why it wouldn't work technically though - `of` is just a filter like all the rest.

I don't see why allowUnknowns doesn't make sense - they won't get the unknown values back (doing that would be difficult, but it would allow them to validate records in a looser way which shouldn't cause problems.