I think there is a better case to be made for honourary then for riddum. Honourary has a much higher occurrence on google, is not pronounce different and is used in academia and government. I personally feel that riddum would be considered by most to be slang while honourary would be more a variant.

I think that in a dispute over whether it is a variant we should error on the side of accepting the variant provided sources can be found. In general I would agree with correcting it to honorary unless it's a Canada article and the sources use honourary. In that case I think the article should go with the predominately sourced one, except in the case of a direct quote.