It's going to depend on the hotel.

On my recent stays where the Diamond breakfast was in the restaurant and not just a buffet, it was a credit per person. So as long as the room service came from the designated restaurant for Diamond breakfast, I could have probably applied my credits to the room service. (Disclaimer: I've never actually tried.) Likely wouldn't cover everything, but it'd knock the cost of room service down to something sane, I suppose.

Since there isn't one standard answer, you'll probably need to phone the property. Or tell us what it is...maybe somebody here has done a recent stay there.