<p>I would like to extend some existing applications' drag and drop behavior, and I'm wondering if there is any way to hack on drag and drop support or changes to drag and drop behavior by monitoring the app's message loop and injecting my own messages.</p>

<p>It would also work to monitor for when a paste operation is executed, basically to create a custom behavior when a control only supports pasting text and an image is pasted.</p>

<p>I'm thinking <a href="http://research.microsoft.com/sn/detours/" rel="nofollow noreferrer">Detours</a> might be my best bet, but one problem is that I would have to write custom code for each app I wanted to extend.  If only Windows was designed with extensibility in mind!</p>

<p>On another note, is there any OS that supports extensibility of this nature?</p>

With jQuery newer than 1.2 you can use
JSONP