Answer: Yes. I would definitely love to have a preview access to XenForo 1.2
It would enable me to test my add-ons even before 1.2 beta is released. Best-case scenario, I would have them updated and working the day 1.2 Beta is available. It will enable me to better serve my customers which in turn are XenForo customers which will be happy about their add-ons being supported earlier on. It will enable users to upgrade faster and if people are waiting for the update to renew their license they will renew faster too.
As far as charging for it. Sure, why not? Other companies do it. Charge a $99 fee for developer preview access, and that requires an existing XenForo license. That way there is no need to cherry pick which people get access to it, and nobody gets without a chance. Only the serious people invested into the platform will go for that.
And as usual, disclaimers of "unsupported" apply everywhere.
Now .. about whether the XF devs would go for that ... I am really unsure. It is extra work. They need to wrap up the release. Package it. Release it. Make sure that only certain people can download it (more complexity on the licensing and downloading system). A major distraction from the bug fixing and that might even make the 1.2 release even later...
What is more important? Earlier add-on compatibility? Or an earlier 1.2 release?