Honestly, Christian, I don't think any of us think it's a question of 'passing off'. I'd have no problem with the figure as sculpted as standing for a Votadinian nobleman. Trousers of some kind would be 'better' but I don't think bare legs is necessarily wrong.
My only problem with the figure as you've painted it is that the chequered pattern as you've done it is a) about 1,000 years too late (it's too complex in other words) and b) inappropriate as it's associated with culturally 'Scots' (Irish) not 'British' (Welsh) groups, from a different part of 'Scotland'. In other words, even if the 'Scots' in Argyll were using tartans like that at that period (and there's no evidence they were), there's nothing to suggest that the British in Lothian or Northumberland would do so.
But a simpler pattern (such as the 'Falkirk Sett') would be appropriate, I think.