My friend has a toilet with a conventional syphon flush mechanism but it seems to have an added on internal overflow that comprises of a float that when it rises it opens a hole in the flush tube. It seems quite heath robinson. There is a metal U shaped band with a rubber lining that is normally closing a hole with a rubber surround in the down tube.
Before we try to adapt something to suit - I wondered if anyone can identify it from the description and suggest a source of a repair kit.