I've asked Hilton through email and they were not able to provide me with email addresses because "they only know the internal hilton email addresses". While a somewhat incredulous answer, they did offer to forward my emails to the property. This was not the Diamond Desk, btw, just a general enquiry through the Hilton website.

What I've done in the past successfully was:
a) Call the property directly and ask what so-and-so's email address is
b) Google the hotel name