For the base, build a sturdy box with a hole wide enough for the pole to fit, meybe a little over 50mm. Really reinforce the hell out of the stage with lots of beams. You can put decorations and padding on it in the end. You can get a metal plate and a metal tubing bigger than 50mm but that will hold a 50mm pole snugly. Cut the metal tube length to be flush to the height of the stage. Weld it to the plate. Bolt this tube plate thing to the stage (inside.) Now you want to get a 50 mm pole. You can just get a roush 50mm piece of steel pole, have it smoothed down and powder coated, or you can buy a piece of polished stainless steel. Either way, get a 50mm pole, drill a couple of hold in the bottom for some "pins." Drill some holes into your tube that is attached to your "plate" that correspond to the holes in your dance poles. Slide the ple into the tube in the stage and place the pins in to lock it in. For additional support, you could try getting a second smaller diameter tube to weld into the stage tube, that is about the width of the inner diameter of you dance pole, so that the dance pole is literally locked into place by an inner and outer wall and a few pins locking into a non rotating position.
Keep in mind, this is just an idea, and I highly recommend consulting a professional on this. Go talk to a fabricating company, a welding company, and a steel copany, and see what they say.