I don't see any issue at all with the franchise directed to those whom the issue affects most of all, ie the residents of Scotland and those who reside elsewhere but have retained an interest as shown by remaining eligible for a postal vote.
If you've moved away for whatever reason and taken your vote with you then that indicates to me where your priorities lie. Anyway given the timescale I'm sure people who feel strongly enough will be able to move back to their country of origin in time to register again if they feel strongly enough about it.
And anyway, what makes someone Scottish, residence or birthplace, red hair or a taste for whisky and salmon?