To my best knowledge, there is no accepted standard for handling user trash. If you know better, I will be happy to look up precice pointers... I spent ten minutes perusing the XDG mailing list archive without success.

As for Drag&Drop, this is another feature that is anything but standardized across Windows Managers (the concept doesn't exist under X11). For a long time, many toolkits (GTK+, QT) just sent a mid-click event at the end of the drag, that was largely seen as a paste request by the targeted client. Now, they use complicated, incompatible mechanism involving atoms and different clipboards, making very hard to catch and interpret them at the xlib level. This (partly) explains why you do not have event notification for it in adesklets, and why it would be hard to do it nowadays, even if I included a Copy&Paste event support.