> What changes would be required to the MacOS installer to declare TkDND ...

Let's get Windows settled first and then I will look at adding it to the macOS installer.

Regarding other platforms: we don't provide binaries for anything other than our own Windows and macOS installers.  So I think there are a dew issues here:

1. What to do about third-party distributors of Python? (That includes macOS and Windows).
   The best we can do, probably, is to make sure the recommended dependency
   is mentioned in the What's New in 3.10 document. There may be some
   appropriate mailing lists or groups to discuss further; I vaguely recall
   there being one someone that was focused on distributors of Python.
   But there will be a *lot* of variability here since not all Python
   distributors also distribute their own version of Tcl/Tk for use
   with Python.

2. We would also want to make sure that our buildbot fleet is updated to
   include TkDND for those buildbots that attempt to do gui tests.