Wow, thanks everyone. I hadn't had a chance to check this thread before now and appreciate the interest and many suggestions.
Briefly, though (and I should have mentioned this), I did try masking the FSS and sending it through a color adjust node. The problem seems to be that in addition to normal displacements the floor is also displaced horizontally with a redirect. So when the masks were applied everything more or less exploded.
The redirected displacements are only needed to add detail to the vertical face of a shelf on the floor (not shown in this frame). Hmm. I wonder if I can constrain those by altitude or slope so they don't affect the horizontal surfaces.
I'll be going through everyone's posts more carefully when I have time later today and will try a few things. If I'm still stuck I'll put together a simplified tgd and post it.
Thanks again. This is such a great forum.