If Scotland could remain in the EU (By remaining, not re-joining, which Madrid would oppose) then they almost certainly would. This means that "brexit" would imply the end of the "United" in the UK and I believe that would be enough of a change in circumstances that a) leave politicians could hold off invoking Article 50 without losing face or b) a referendum for the new conditions could be held.
So the Scottish Parliament don't really need to veto Brexit to stop it, they just need to threaten to leave the UK and remain in the EU to d oso.
The new conditions of Brexit (which really were known before but likely not phrased as such) are "Should England and Wales leave the UK and form the United Kingdom of England and Wales, and then leave the EU?". I doubt that's a very popular idea.