And this is a question about the most famous koan collection, which means that the so-called AI had many materials to be trained on, even in English! How many people would find this plausible and believe it?

That would be fun to do.

It would indeed link two very famous masters in one calligraphy. It must have been very tempting to believe. And if by chance it were true, that would be make it priceless!!

nothing less than

The Genkoshakusho 元亨釈書 (sorry, another Japanese source), tell us that Wuxue did in fact studied under Xutang.
He is a good candidate for Librarian Yuan...That would make the calligraphy even more valuable (assuming it is not one of the countless forgeries)

He may be nothing less that 無学祖元. At least that is what this japanese edition of the 虚堂録 is saying in the notes.

saku.hanazono.ac.jp/database/kid...

Incredible!! I wonder who is this XXX元. No one with this character among his heirs of Law according to the 禅学大辞典