It's not a matter of stress, it's a matter of bonding area. Soft solder needs enough area to get a strong bond with. Shotgun ribs are wide enough and long enough that soft solder will hold just fine. In fact, some ribs are glued on with epoxy or Loc-Tite-type bonders.
The magwell butts against the bottom of the frame, and this just isn't enough area to hold very well. You could try it, but the recoil or a good bump will probably knock it loose.
In my experience, solder blackening agents work only on small touch up jobs, and not too great then.
