From what I read La Beltraneja was married to the Duke of Guienne before her marriage to her uncle, if this marriage is consummated and produces children, would one of her daughters marry Arthur Tudor or Henry VIII..
No,there's no benefit of doing so.Unless Arthur is still alive,and Henry,as the spare,decides to go on an adventure in Spain to claim the Castillian throne.
But any daughter of Joanne of Castille is more senior lancastrian than Isabella.
The eldest son of La Beltraneja would be the possible 2nd husband of Anne of Brittany, Anne of Brittany would bring with her not just Brittany but claims to Navarre as well..If Charles duke of Guyenne and Joan the Beltraneja have children, then you have other questions.
Does Charles live much longer (he was 26 in 1472) or not ? If yes, then he lives to his late fifties or early sixties, then he becomes king of France in 1498.
If Charles and Joan of Castile have children, do they have a son or only daughters ?
If they have a son (who could be born by 1480/1485), then this son becomes king of France either in 1498 or later (if Charles of Guyenne lives long enough to become king himself).
And then there is the question of the claim to Castile's crown.
OTL, Joan had the support of Portugal (the most important) and of France (quite minor because Louis XI could not distract a lot of resources because of his cold war against Charles the Bold of Burgundy).
If, Joan is married with Charles of Guyenne at the time of the conflict against her half sister, she will probably not have Portugal's support.
So will Joan and Charles of Guyenne be able to prevail in the Castilan war of succession ? Not sure at all.
If she can't prevail as OTL, she will just retain a theretical claim to the throne of Castile as OTL.
And if Joan and Charles have but daughters, then their children will lose the throne of France. And you still have the same difficulty for them to prevail in Castile.