Indeed the download web page implies the OS is not supported, but I had the dmg by downloading from a different computer. After mounting the dmg and running the installer app, the app would run, I'd click the 'accept' button. get prompted for and enter my user account password, and then the installer would just sit there forever with no progress (I left it running for hours). I then force-quit the installer and restarted the OS (which would hang during that shutdown, requiring a power-cycle).
This morning I viewed the Troubleshoot Adobe AIR installation for Mac OS and then deleted the /Library/Frameworks/Adobe AIR.framework directory prior to trying again, and this time the installation worked. Apologies for not doing my homework and reading that article first. I was trying deleting traces of AIR under my user directory prior to trying to install, but wasn't aware of the directory above (doh!). I should have tried a recursive find from the terminal, or just read your troubleshooting document.
FWIW I had the previous version of AIR (26) installed prior to the OS upgrade, and tried to install 27 after the upgrade, and then hit upon this problem. Perhaps it was just a fluke.