I leveled the master bedroom ceiling in our house the way you are talking about--sistered light steel studs onto the sides of the joists. Clearly, nowhere near the area you are talking, and I was also working with standard 2x10 roof joists (flat roof), 16" OC.
The procedure was set the level line using a laser, and then checking for maximum (somewhere around 3"--in my case actual sag plus a section where water damage had caused some problems--and much to hard to correct because of the way this house was built.)
Not too difficult technically--I used screws to attach studs and used a tighter than standard spacing on sheetrock screws on 5/8" board in the field as well. I now have a pretty much dead flat ceiling and no one will know that it is a bit of a fake job, since the roof will (eventually) be redone using roof board built up. Settling does not (IME) mean that the building is going to fall down, if you have some idea of the root cause.
It is also pretty cheap to do that way as compared to a full suspended grid, since you can pick up steel stud pretty much anywhere (in commercial lengths if you can handle them--I used 8' since I work by myself).
The question though in your case will be the weight load on the bottom chords--it may be worth it to spend a few bucks and get an engineer to look at it.
Also as has been said, the 24" span (partially addressed by using 5/8" I would think) and (likely) 2x4 bottom chords may require at the least strongbacks, and possibly addition bracing.
Our local guy does small jobs like this, and charges less that you would think (we just had him design a steel and paralam set-up to open up a kitchen we were working on--I think it was only a few hundred dollars.)
