Have the Lancasterian dukes be given the duchy of Gascony by Yorkist-line kings of England.
Simple.
And by that virtue the Lancastrians become vassals of the king of France without being kings in their own right, which simplifies matters.
Of course if the Lancastrians lose their eponymous land they'd have no obligation to England.
How about Charles VIII of France decides to intervene so when the Lancastrians lose the War of the Roses he recognises them as the true rulers of England and re-grants whoever the leading claimant is the Duchy of Gascony as a fiefdom in return for them paying him homage? Anglo-French relations don't seem to of been all that great in this period so having their own pet-pretender to the English throne in their back pocket to cause trouble could be useful.
How about Charles VIII of France decides to intervene so when the Lancastrians lose the War of the Roses he recognises them as the true rulers of England and re-grants whoever the leading claimant is the Duchy of Gascony as a fiefdom in return for them paying him homage? Anglo-French relations don't seem to of been all that great in this period so having their own pet-pretender to the English throne in their back pocket to cause trouble could be useful.
Something like that. If the alt-War of the Roses drags on for roughly the same length as our timelines then only a decade or so later the Italian Wars kick off and France and England were mostly on opposing sides IIRC. Plus if a half viable candidate remains at large in France would there be a marriage between the two houses, and how accepted would it be? Even in our timeline where Henry VII was solidly in power and had reunited the houses there were still several rebellions. Having a pretender to keep the English on their toes for the cost of a small part of the French Crown's land seems like a bonus, and if they ever get tired of them they can always just trump up some sort of excuse to take it back as it's not like the Lancastrians will have any major outside supporters.So Charles VIII does a Louis XIV two centuries early?
How about Charles VIII of France decides to intervene so when the Lancastrians lose the War of the Roses he recognises them as the true rulers of England and re-grants whoever the leading claimant is the Duchy of Gascony as a fiefdom in return for them paying him homage? Anglo-French relations don't seem to of been all that great in this period so having their own pet-pretender to the English throne in their back pocket to cause trouble could be useful.