Whatever I said, it never hurts to just come to me (or any developer) and simply ask.
Hey, what are your plans for this, I'd love to see that on XenForo? .. Do you have any intention in making it, have you already started? Could you use some help? Mind if I give it a try, but give you full credits, etc?
A gentleman agreement after simply asking first, will get users : very far. It shows a bit of respect, rather than disrespecting the authors by just taking whatever they made, revamping it for xenforo and calling it their own. Even if it is a complete rewrite.
I rather get asked, and do some teamwork to achieve something that benefits everybody - and everybody's on the same page. Than having to deal with some douche that didn't ask and perhaps ruins it for everybody.
People are free to discuss it on this site, but sure, xenfans.com is also a great place to discuss anything community related, or particularly xenforo related.