I think the only restaurant that validates Valet is Emeril's. Yes you can valet for the whole day. I believe the cost is around $16.00. Preferred Parking is $3.00 more than regular parking. I personally do not think it is worth it.
From the USF site:
Garage Parking:
Universal Orlando's state-of-the-art enclosed parking garage is complete with protection from the elements, a 200-camera security system, moving sidewalks, escalators and elevators, on-site wheelchair rental and an on-site kennel. Vehicles can leave the garage and return the same day by stopping at the toll plaza upon re-entry and presenting their parking ticket.
Daily parking fees per vehicle:
Cars, vans and motorcycles: $8.00*
RVs and trailers: $10.00*
*Prices are subject to change without notice.
Valet Parking:
Valet parking is available for cars and vans at a charge of $16.00*. Follow the signs to valet parking.
*Prices are subject to change without notice