Oh, I misunderstood. Personally I wouldn’t want to evaluate that through multiple associated tables at runtime on every request. Just store it. Create a new field in the table (e.g. title). Whenever the category-ID or type-ID changes update that field on the save. Set that field as displayField. But I’m not sure what we are doing here and why.
If that is the problem you want solve I say you’re looking for the wrong solution with displayField. This should be possible through ->find(... without all this trickery.