Thought about that, but I am just in love with how they will look stacked on the back...Maybe you could tuck them partially under the seat instead of directly off of the back. I'm sure that with your metal stills that you could transition the horizontal tubing for the lights into the curved bottom seat rails.
Here is a mock up of both lights side by side down low on the seat:
Here is a mock up with the lights stacked on the back of the seat with the current mounts, I have taken to calling this mock up "The Batmobile..." Imagine it all done in the metalflake, wheel centres painted grey with chrome trims...
There is no doubt that if it goes over backwards they will be damaged, but with the new lower handle bars that is far more unlikely at least...
Is it worth the risk? It looks like if they are damaged it would only be the lenses which are available separately and reasonably cheap compared with the bases...
Luke.