TkXext "has the ability to send events to any window, embed windows in Tk,
and send events to embedded or non-embedded windows."  Unlike [Android], it doesn't
yet (as of September 2001) have capabilities for dispinfo or
event recording.

[GPS] maintains TkXext at http://www.xmission.com/~georgeps/tkxext/ , with the help
of several others.

TkXext is X-based; it's available only for Unix.