I'm not sure how this would happen, I figure Scotland changed ruling dynasties at least twice (from Dunkeld to Bruce, and Bruce to Stewart) while the Plantagenets ruled England, and there were quite a few Plantagenet princesses who became queen-consort of Scots.
One of the parameters of this challenge is that Scotland must remain a separate, independent nation, so the Plantagenet line that comes to rule it descends from a younger son/brother of the king of England - for whatever reason (I particularly like the accidental succession-idea).
And also, the Scottish Plantagenets mustn't keep needing to call in their English cousins to bring an army and stamp out dissent, so they sort of need to be acceptable/palatable to the Scots. IDK if this is even possible, but I look forward to seeing if you guys could find a way to make it happpen.