Would it be sufficient to:

1) Restrict collections.Set()'s operators to accept collection.Set but not arbitrary iterables, and
2) Fix Issue2226 and let set() | MySimpleSet() work via collections.Set.__ror__

Attached is a patch that implements this approach, nominally fixing both this and Issue2226.

This solutions seems much too simple in light of how long I've been thinking about these bugs.  I suspect there are code hobgoblins waiting to ambush me. ;)