Would anyone consider the old unofficial Orkney flag, which also happened to be the same as the flag of the Kalmar Union of Nordic countries? It's fair to say Norn speakers in Orkney, Shetland and Caithness were once represented by it:
I agree that it would be nice to have something like a flag. Scots Gaelic speakers like myself tend to just use the Scottish flag, but that causes problems when the Scots language needs representation too.