yes, that's what I had meant to convey when I said an advantage of flying out of Spain is that, unlike flying out of the UK, almost all your dollar spend is SQD. E.g. flying PY from Madrid to SFO return is 91 euros in 'taxes, fees and other charges', whereas ex FRA it's 193 euro and ex-Heathrow it's 350 euro (exact amount may vary with routing but this pattern holds in my experience).

Why I'm interested to know now though is whether I could get an EDI-MAD positioning flight as SQD, using the mechanism Quail mentioned? And more generally, is this a way of getting intra-Euro flights into SQD, as long as there is some domestic Canada segment you can fly later?