Main structural design problems relate to
(a) local locker loading .. shear and bending in the main
(b) (for larger, longer aircraft) fuselage bending loads
The original certification will have included anything which is mandatory equipment. It follows that if you stick something else in the area then, without specific certification reassessment, the usable locker freight load goes down by the mass of the modification.
One of the problems is that a lot of "little" mods get done without the level of attention to paperwork detail which the system anticipates.
You would need to go back through the aircraft history to get an accurate answer to your concern and then the remaining available usable load should fall out of the review without too much trouble.