Despite evidence to the contrary, the temporary MacOS.WMAvailable code showed that there was *not* a window manager connection.  I'm still not sure what the difference in environments is (perhaps it is just due to a different version of Tcl/Tk on the buildbot) nor why the buildbot only fails intermittently.  In any case, I've taken Ronald's suggestion and replaced the Tk-in-a-subprocess sanity check with a ctypes version of what MacOS.WMAvailable did.  Let's see what how well that works.