I wonder what drilling through the tri-clad would do. If you weld you would need to weld both to the bottom and top and that could cause some heat stress and de-lamination of the layers.
Soldering might bond the layers but I also wonder what the heating would do - much lower local temp but more global heat added. I guess we wouldn’t know until someone does a few.
Regarding Bobby’s comment... I think the industry is still ~2/3 propane, at least according to John Blichmann. Ironically his kettles aren’t tri-clad, but he doesn’t see the value in bottom drains.
I guarantee someone will create a HB scale kettle with a bottom drain, then be like “look what a great idea we are bringing to market!”