Yep, I know exactly what you are thinking. I have two stems on one of my bikes, with a kiddy seat (LOCT) on the front of the bike. I have a second pair of bars (cut down carbon road bars into a stubby flatbar) for my daughter to hang onto. I used a BBB steerer extender and cannot fault the strength, but of course the weight is not a factor on this bike - 6" travel freeride bike with beef for 1.5 riders
I would not weld anything to a set of bars. Too critical an item to mess with, and too many variables to be stronger than original.
Why don't you want to use a steerer extender?