I've stayed on a day rate here before. You can both call ahead or book at the front desk upon arrival. I'd recommend calling ahead. I've booked day rates via email for other Hyatt hotels as well.

You might want to verify day rate hours though, since I doubt it will cover all of your layover time.

I'm not sure if breakfast is included for Diamonds. I recall at another Hyatt that it was not, but I am uncertain if this is a chain-wide policy.