Mike,
If I understand you correctly, your considering permanently attaching the domino tenon into a mortice in the headboard/footboard leaving the stub sticking out, then joining a bed rail with a corresponding domino mortice on its end, then inserting a screw through the inside wall of the bedrail into the stub tenon to secure the bed rail to the headboard/footboard. My concern is how you ensure that the bed rail is drawn up tight against the headboard/footboard so the joint cannot rack slightly when in use (children jumping on the bed?). I would use at least two dominos spaced apart in each end of each rail; the deeper the rail, the less likely it will rack relative to the headboard/footboard.
Most hardware systems for joining bed rails to a headboard/footboard have a mechanism or tapered shape that draws the joint tighter together as the load on the bed increases. Others simply use bed bolts extending though the headboard/footboard into the bed rail which can be drawn up very tight.
Have you considered reversing your proposed arrangement? If I was wanting to use domino tenons to join a bed rail, I would glue a pair of them spaced apart in the end of the rail, make corresponding mortices (sockets) in the headboard/footboard, then insert a decorative head screw through the outside of the headboard/footboard into the rail. To help ensure that screw does not pull out, I would insert a domino through the inside surface of the rails (not penetrating all the way through the thickness of the rail) so the screw will go through the cross grain of the domino when the screw is inserted. Cross grain holds screws and nails better than end grain. Also, driving a screw parallel to the layers of plywood without a pilot hole is likely to cause splitting,
I built a bed of hard maple for my son ~20 years ago that is still in daily use and is as solid as any ever made. The rails are quite deep (about 17 inches) because they also house a bank of three deep, full width drawers above a full size trundle bed drawer. The ends of the rails fit directly into a stopped groove (dado) in the headboard/footboard, and decorative head machine thread screws extend through the headboard/footboard into threaded metal inserts I installed in the ends of the bed rails. The bottom of the stopped dado aids in positioning the rail in the right position when the screws are being inserted or removed rather than crashing down on the floor or your foot.
If I ever build another bed, I will probably use a pair of 10mm dominos at each rail to headboard/footboard joint together with a decorative head screw which extends through the headboard/footboard into the rails. And I'll probably install threaded metal inserts in those rails for a connection that will last for generations, and not likely to loosen up.
Dave R.
