Attempting Indoor Water Use Reduction Credit using minimal plumbing fixtures that serve occupants outside of the project space

I seem to have a unique problem.  My project is a lab/office space that is occupying half of a floor in a building.  It seems no new plumbing fixtures will be installed for the existing public restrooms that are situated inside the project space, the only thing that is new inside the project space is the pantry sink.  However, our scope also includes a new lactation room with a sink, and new gender neutral restroom with a sink and toilet, to be designed outside of the project space!  Meaning both rooms will be utilized by everyone on that floor, even if they do not occupy the project space.  Is it still possible to pursue this credit considering the project space only has one new sink for the pantry and the remainder of the new plumbing fixtures will be in rooms outside of the project space, while also dealing with existing plumbing fixtures from the existing public restrooms in the project space?  Would we have to estimate the FTE occupants from inside the project space AND outside the project space?  Would we still need to include the existing plumbing fixtures in the existing public restrooms inside the project space, even though if it is not part of the scope then we wouldn't need to include them (as per the indoor water use reduction prerequisite)?  Is this approach even possible since we're only considering the outside lactation room and gender neutral restroom, and pantry sink located in the project space for new plumbing fixtures?  Any advice would be greatly appreciated since we are currently below LEED Silver, and the client's sustainability guidelines require no less than LEED Silver.