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.