Aerosol block out/filler for screen printing?
I don't know of any spray on screen filler but you may want to look into trying some light sensitive photo emulsion.  Its kind of like taking a photograph, basically you cover the screen with the emulsion, let it dry, place the leaves on top of the emulsion, and expose it to light.  The light will harden the emulsion all over the screen except where the leaves have blocked out the light.  You wash/scrub the remaining emulsion out of unexposed spots.  Since the emulsion is light sensitive, you'll need to keep it in the dark until your ready to expose it, any room with out windows, or with the light completely blocked out will work.  You'll also need to use a red light while working with the emulsion in the dark.  I've used Diazo photo emulsion but there are plenty of brands out there.  You can find it online or at one of the chain arts and crafts stores.  Its not too tough to do, this is how I make my screens.

Here's a link to more detailed instructions from the manufacturer.  

http://www.jerrysartarama.com/DOWNLOADS/INSTRUCTIONS/DIAZO%20Instructions.pdf