To incorporate the detent pin assembly, the steel tube would need to be fairly thick, adding to the weight of the shotgun ... in the wrong place. I think I would try moving the detent pin assembly away from the magazine tube. Might be able to get it far enough from the steel tube but it would be visible and not concealed by magazine cap. Someone with a machine shop could probably make a wider magazine cap. These are solutions that I'm sure Val Browning could have worked out when he was perfecting the A5 design (changing safety, adding auto load 2-piece follower and magazine cutoff). But I guess it was better to just keep selling replacement fore ends. Keep in mind that back in the day Browning pretty much had the autoloader shotgun market cornered.
I agree that the detent housing would be tough to hide completely, but where that pin is located is pretty concealed. Anyone examining the gun closely enough to see it would probably view it as a feature rather than a problem. As for the balance issue, in my specific case, the barrel is 24" long, so I could probably duct tape fishing weights to the muzzle and have a decently handy gun. For a 28"+ barrel the balance might come up more. There is also the issue that when I miss stuff with a shotgun, it's rarely if ever the fault of the gun.