OS X own Apple Remote support will put your Mac to sleep when you press and hold the Play/Pause (Select) button. That’s also what the Harmony will do: send a long/held press of the Play/Pause (Select) button.

This will only work in moments where NO applications on your Mac are using the Apple Remote and OS X default handling is in charge.

To put your Mac to sleep on a press and hold of the Play/Pause (Select) button when using Remote Buddy, go to: 
1) Prefs > Mapping > Global Mapping > Apple Remote 
2) Select „Miscellaneous > Put computer to sleep“ for the button in the center of the navigation ring (named "Play/Pause (held)“ or „Select (held)“ depending on Apple Remote model/Remote Buddy version).

Please note that this action is then „hard-wired“ to this button code and you can’t use that button code for anything else / on a per-application basis. Pressing and holding that button will then put your Mac to sleep regardless of whether you manually pressed that button - or the Harmony sent that button code on a press of „Power Off“.

Best regards, 
Felix Schwarz