Earlier I noted: "Somewhat surprisingly, the segfault doesn't seem to happen under the same conditions with 2.7".  I'm not sure now how I came to that conclusion then but it is incorrect: the segfault definitely also occurs under the same conditions with 2.7, at least when using the current ActiveTcl 8.5.15.  It did not seem to fail with the Apple system Tk 8.5.9, a more plausible result than not failing at all with 2.7.  Next step: test with various Cocoa Tk versions.