There's a "hack" that uses pmset to immediately sends your displays to 
sleep for dashboard and as a seperate app:

http://linestreet.googlepages.com/home

You can simply the app version of it to the Remote Buddy menu by 
dragging it into a place of your choice in the menu.

I chose to say "hack", because I've read a few comments here and there 
that it would not work as expected on *some* machines, effectively 
keeping the displays from waking up again until the next reboot. 
Remote Buddy has no direct support for display sleep for the same 
reason. However, the tool worked nicely the last time I tested it.

Best regards, 
Felix