Depending on your circumstances, you may be able to run a chain, or a cable from the roof to your window box. This will make the water from the roof flow down the chain / cable to the window box. You may have to use mutiples depending on the size of the planter.

Thanks for the help, jduke
I'll try something like this.
Interesting idea!

CDO and Doug

I found a few links after searching online for this.

http://www.rainchains.com/

http://www.gardenwinds.com/product_info.php?products_id=387