Interesting. I just finished updating a part of our system to manage selecting testing accommodations in our database.
I think that you have oversimplified your explanation as (at least here in California), Testing Accommodations are not specified only for 504 (physical challenges) but also for various learning challenges specified in a regular IEP.
But if you are using FileMaker 13, I'd stack two objects, one on top of the other and use reciprocal "hide object when" expressions to make either the layout text: "N/A" or your value list formatted field of accommodations visible depending on which should be visible for a given student record.
PS. I used a filtered selection portal inside a popover for building a list of accommodation statements for each of two different tests in my employer's system. I built on the concepts described for selection portals inside popovers found in:
I didn't need the technique that I am describing here as we use a related record on a dedicated layout for students with special needs so one doesn't even get to the layout where you can select this unless the student already has been identified as having the necessary special needs designation and associated IEP. (Individual Education Plan)