It's usually possible to assign commands to the "Power on" and "Power off" events in the Harmony. I've tried this with the Harmony Smart Control and unfortunately so far it doesn't seem that these get sent by the Harmony for a "Sony PS3" device.

I assume that this is a bug in the Harmony Hub's software and will keep an eye on this as Logitech updates their firmware.

Until then, what you can do is dedicate a button to put the Mac to sleep. For this, what you'd do is to go to "RB Prefs > Mapping > Global mapping" and select the action "Miscellaneous > Put computer to sleep" for the button you want to dedicate to putting your Mac to sleep.

You can then use that button to put your Mac to sleep as long as the Remote Buddy activity is active.

Best regards, 
Felix Schwarz