When your Harmony 880 is able to fully (that includes long presses, 
not just a replay of a short press) emulate an Apple Remote, you can 
use it with Remote Buddy just like an Apple Remote and use all of the 
usual features. You could map a "Put computer to sleep" action to a 
button on a per-app basis or hardwire it to a button (=> find step by 
step instructions on both in the Remote Buddy Help that's included 
with Remote Buddy).

If you have trouble with setting up your Harmony 880 however, please 
contact Logitech's support for assistance. Maybe others on the forum 
can help you, too. Not owning a 880 myself, I personally can't help 
you with that.

Best regards, 
Felix Schwarz