I'm letting the Hilton Internet Reservation Customer Service folks argue with the hotel. The issue is that the Diamond Club membership requirement isn't explicitly stated on the web site as it might be for an AAA or similar rate.
So far they've been getting nowhere with the duty manager who apparently wants to call me directly and negotiate a rate since I'm a repeat customer "who knows about the club." Well, though I am a repeat customer, I wasn't aware of the Diamond Club until now.
Hilton internet CS says they will talk to the hotel's general manager and try to get them to honor the rate as advertised. In the meantime I'll make a backup booking.
For any Hilton folks reading this, I am quite pleased with the advocacy shown by the Internet CS reps. (I gave the rep an "Atta boy" through his supervisor.)
Rich