Dave,
In regard to the idea of a detachable magazine tube on a shotgun, there was one conversion kit in the early to mid 1990's for the Mossberg model 500 shotgun. I believe that it was made for the 500 with the extended magazine (I think it was 7 in the tube, and not usable on the 590).
The mag tube would be pulled out for a reload when the fore end was racked back. But I don't know what kept it from detaching when cycling the action. The kit had two magazine tubes and probably some sort of part to allow them to lock and unlock from the receiver. It also had a fairly steep price in the 200-250 range. I would look for the Shotgun News ad it was in, but nearly all of my older ones were thrown when I moved. It was a nice idea, but detachable mag tubes are like those USA readi-loaders in having the deficiency of being an awkward size for carrying and manipulating.