Peter,
You did not say if the service is a separate service or not. Most elevator rooms I've done are so we will assume this one is also.
You can install an equipment ground in it's own conduit. Yup it raises the impedance WAY high in this case but don't forget, it plays almost no role in fault clearing at any rate, it's only there as a 0 point reference.
My big question is WHY do you need one running all the way back down to, I assume, the building grounding electrode from the 23rd floor? Is there no building steel? In lieu of that, is there no grounding electrode in the closets for xfrmr reference?
If elevator equipment is that sensitive, this stuff is doomed anyway, or are the elevator people becoming like computer people, you know "Our stuff is special".
Good luck.
