hi Sim,
...not sure if I got it right, but if you have a custom field, in a record, which is generated by a script(for example, on recordLoad), then that field is not accessible by SavedSearch.
Only fields which are defined in the record(definition; standard fields+custom fields) can be reachable in a SavedSearch.
(you can try in UI and if smth is not reachable in UI, for a SavedSrch, then it won't be possible any other way...)