Remote Buddy allows you to freely map any button to an action, 
including, if you so want, hardwiring a long press on the play button 
to system sleep.

As, however, with only six buttons, the number of functions you can 
map to the remote are already kind of limited, the default 
configuration is to give you application specific functionality for 
the "play hold" state (mostly used to toggling to fullscreen or 
another special mode).

You can map the sleep function (found in Miscellaneous group of the 
popup in the table) in Preferences > Hardware > Apple Remote > Mapping.

Then, a long press on Play will always put your system to sleep. 
This, of course, will also mean you can't map any other function to it.

Please have a look at the online help if you want to learn about how 
actions for button presses are chosen.

Best regards, 
Felix