I took a quick look at this and my impression is that clipboard support
must be explicitly added to the TkAgg backend and AFIK has not been. My
guess is that this is a wider reaching problem than TkAgg and it should
go into the list of future enhancements for matplotlib.
It sounds more complicated than I thought. Indeed I was searching for existing solutions that would copy the window contents into clipboard. I know it is quite easy to do with Tcl/Tk and the BLT extension or something similar. You just call a function with the window path as arg and your clipboard is filled. This is why I focused on TkAgg.