A gravel filled bag should work just fine. As long as there's surface area and water flow around it, it'll harbor beneficial bacteria. Some may argue not as much, but if it keeps ammonia and nitrite at zero, then it harbors enough.
Not to Hijack your thread, but I actually did this with my Top fin 60 on a 55gal. Bought two mesh bags almost as wide as each chamber, filled them with Fluval Ceramic rings, (your gravel idea would work just as well in my mind) then placed them after my carbon filled floss bags. Water tests say its working!