Here are my thoughts. An engineer somewhere decided how long the PB inner cable and the cable housing should be. If you increase the length of the cable housing between the anchor points you will in effect, shorten the remaining balance of the inner cable. By doing this, the extra slack in the cable you now have will be less. I don't know if it's enough of a difference to make it operable, but I believe it's worth a try. I would certainly try it before you modify any of your perfectly good PB components.
See the inked version of your photo. The blue arrow is where the end of cable housing is now. Relocate it to the rear somewhere between the clamp bolt and the red arrow. By taking the slack out of the inner cable the PB lever should move without hitting the backing plate.
