making contents of other programs transparent to Emacs

[Joe Corneli]

This seems like a hard question, and not completely Emacs-specific.

Suppose I wanted to make the contents of e.g. an rxvt or Mozilla
window readable to an Emacs.  What would I need to do?  The text is
available to X (and I can copy and paste it into other programs) so
it also seems that with more work I should be able to treat the text
in these other programs as a read-only Emacs buffer.