Natually, this is a question that most directly affects those currently residing in Scotland. But I share your view that Scots not currently in Scotland should have a voice too.However, my view is that this is rather unfair. This is a constitutional question, not something merely about local living conditions. UK citizens who had partaken in Freedom of Movement and moved into the EU were effectively disenfranchised about something which directly affected them and their security. Scots moving to England are in the same position
Since UK referenda are non-binding, perhaps an idea is to extend registration to those who are not resident in Scotland, but who would be eligible to vote in a UK general election, but count their votes separately (e.g. same question, different colour ballot paper). That would let them have a voice while still allowing the Government to know exactly what the current residents want (avoiding what some would see as the 'spoiler' effect of the overseas vote).