Dave,
I agree it would be sensible to add OO. ?The difference between OO and HO is even more than that between British N and what could be ambiguously termed continental N without specifying which continents. ?I had a quick look at dispatcher. ?It uses 76.2 for OO. I suspect the precision is not needed for this purpose, but it might be sensible to use the same ratio. ?I guess one could argue that scale is an attribute of the layout rather than a specific tool but ?I realise that that level of rationalisation not likely to happen! ?Dispatcher also designates British N as 'UK N' and I'm not sure that there's a commonly accepted short code for this but that doesn't help your request for a letter pair.
Paul
