11R driver 1.2.12 working with High Sierra
 

Downloaded the High Sierra Beta onto an extra disk. 11R driver 1.2.9 was on there from a previous Mountain Lion experiment. Had to use the 1.2.12 uninstaller to remove it, because the 1.2.9 uninstaller threw an error message. But once removed, 1.2.12 loaded just fine.

As a side note, the "About this Mac" screen running under El Capitan still shows "Mac Pro (Early 2009)" even after the firmware update to 5,1 (as expected). But on High Sierra, the "About this Mac" screen shows "Mac Pro (Mid 2010)".

Now what should I try next...