I have used something like these in the past:
So for a corner joint - table leg to 'box' frame - what about drilling a very tight hole in the corner of the leg and *tightly* screwing in an M8 stud (threaded rod)? Would this hold? The exposed end of the stub would be about 6cm long to pass through a diagonal bracket (wood or metal) and the table leg pulled tightly into the frame with a nut and washer on the end of the stud.
MM