I keep a quarantine up all the time that I *could* keep frags in. I have macro algaes, live rock, and sand in my quarantine. I only treat the tank with praziquantel when I have a fish in quarantine, which in my experience is not harmful to most corals ( I have seen some problems with mushrooms). If I need to treat a fish with any other medications I move them to a hospital tank.
Technically, a quarantine tank does NOT need to be a treatment tank, but rather simply an observation (so you can watch the animal for disease) and adjustment tank. A good quarantine should screen for disease as well as help the fish recovery from capture and adjust to prepared foods prior to having to compete with the other fish that are already established in your DT. So long as you can remove the fish to another tank for disease treatment you can keep corals in your quarantine, though it is easier to do them separately in my opinion.