Thanks Frances for your advice when I had my twice-yearly hour in the Manx Museum library last week. The person on duty was a bit reluctant to bring out the old registers (not 20th century Registered Copy), and although I got a negative result from looking at least I can tick that query off.
It is disappointing that the opportunity hasn't (yet?) been taken to put the actual images of the old registers on line, rather than a transcription. In my ignorance I would have thought it would take less time to photograph than transcribe. It occurred to me that maybe they should make it a condition that if you request an old register you have to take digital photographs of it for the i-museum. This would also gradually reduce the need for the actual documents to be handled by researchers.
On my October visit I'll have to try and wangle a bit more time as I'd really like to start looking into property ownership next.