I think there is more inward pressure put on the bows by the mainsheet loads transmitted through the forestay than placed on the bows by the spin. I guess the addition of the spin loads to the mainsheet tension could be an issue but generally the mainsheet isn't on as hard downwind with the spin as it is upwind.
also, if the boat is meant to have a jib then the angle of the forestay/bridle wires to the hulls will be less so the boat has probably been designed to take that load. On boats that were never meant to have a jib there is generally a much higher meeting of the bridle wires and forestay (like the HT). These boats have more of a problem with the inward loads by a lower lying bridle set up- but they handle spin loads fine.
This is from my experience and having had an issue with structural damage from 2 dudes on the wire at the back of the boat and max mainsheet tension. This was without the spin up.
engineering guys could chime in.