Another reason why a "salute cannon" may not be rated for projectiles is simply physical construction. Adding a projectile in front of a charge of powder dramatically increases pressure in the chamber, and a cannon that is safe for salute charges may very well become an accident waiting to happen if the pressures inside increase. I would advise following the manufacturer's recommendations on such matters.
Remember, in the early days of cannon, the failure rate was still somewhat high, and being anywhere in the area when a cannon did fail was not good for ones health.
Stay safe.