if you have the possibility to have something rigid that you can use as form (plug, call it as you prefer) it might work even on a wide surface. You can put together a steam box made with foam, the pink kind, it will work and will let you steam wide pieces.
Avoiding the cracks could be as much luck as technique actually, a void or a joint in the core could result in a crack of the face veneers for example.
Not in luan but in poplar steam bending, or wetting then going at it with an heat gun, worked well for me in airplane rc models, the pieces are smaller but the radius of the bends are tighter. If it delaminates with steam why dont you just recuperate the various plys and put them back together cold molding style?
rather than scoring one face id eliminate it completely.
Maybe you would want to look at "bending" plywood, might save you some time.