The Diamond Desk frequently adds a line to my reservation comments along the lines of:

HH DIA, REQ UPG IF AVAIL

It doesn't always work, and I'm unsure if the front desk knows it came from the Diamond Desk, so I leave it alone.
NJRob, thanks for the insight.

I've also noticed a strange code often ends up in there when I check HHonors.com the day of arrival. I presume that's a trigger the front desk has seen the reservation.