Here's a link that gives some idea about the stresses Boeing engineers were thinking about when they design an aircraft's vertical fin...So long as you're not abrupt with th flight controls, a forward slip is well within the design limits. Which answers the intent of this post. Design limits can be exceeded when "Over Yaw" is induced by abrupt inputs to the rudder. What's far worse is to abruptly cycle the rudder in both directions. However, I think a Boeing would BEND but not BREAK with such inputs. Just a guess. But perhaps, Bugg Smasher, our resident test pilot will try this one on for us on his next trip.
It's reassuring to know that Boeing aircraft are designed to handle full rudder (steady state sideslip) in one direction, followed by full deflection in the opposite direction from steady state sideslip times a factor of 150%!!! That's one tough bird. What must be avoided is aburptness and the resulting "Over Yaw" that being abrupt creates. It would be Gameover on an Airbus. I believe an Airbus is designed to handle the stress of sideslip in one direction only.
http://www.ifalpa.org/sab/03SAB001_U...n%20Boeing.pdf