If I may, I think the logic is flawed here. Madiran and Irouleguy have no real need for a cross between 2 grape varieties that are both permitted within their appellation laws (and in high amounts). Moreover, at the time of the creation of this variety, they were poor backwaters where vines were being grubbed up. Its unlikely there was any investment in them.
Indeed, Bordeaux has a history of adulterating its wines with the more ‘robust’, and more regularly ripe ones from upstream- particularly Madiran, Cahors and Gaillac.
I found it quite funny that when the newly permitted grape varieties were announced for Bordeaux last year, amongst a host of workaday varieties Petit Manseng was inserted. A variety that has made some of Frances great sweet wines for centuries. It was a bit like the local car dealer casually mentioning he was selling a Ferrari amongst his usual Peugeots and Volkswagens.
I suspect that Bordeaux is more likely where Arinarnoa is hiding in plain sight - though now handily has no need to hide anymore.