Actually, that's not so simple. In filemaker, you can define a value list to list all the values in the table quite easily. Setting it up to list all the values in the current found set would be tricky as there's no way that I know of to set up a stored, indexed field that returns a different value for those records currently in the found set.
You can probably set something up that will work, if you rely on a script to pull up the records in your found set. The same script could then set a value a field that in turn matches all the records via a relationship and then your standard conditional value list approach may serve. This gets extra tricky if you want to be able to omit records or create new records and have them also be part of this "set" of matching records that serve as your value list source.