hmm, why woudnt you just want a new window in that specific case? click link get a new window, move it to the side?
not trying to kill your idea, just trying to understand how it differs.
Like maybe its the float on top part. that you want it visible on top of the mail client all the time? regardless of using the client.
if so, im not sure how it could be done without introducing an extra click somewhere in the procces, unless you wanted it as default that it float on top