If i get you right, you have a content type with node reference field, so when you refer a node instead of having node suggestions directly you want them to filter down by provided content type name.
Just to make sure you did'n miss this while adding the node reference fields you will get the option of choosing the content type as "Content types that can be referenced",this is one layer where you can filter/limit the node suggestions.
But yes there are chances when you want to filter the suggestions on the fly,which will be interesting to have.
here is my view for implementing this-
- Build a custom autocomplete field ,backend for this can be served by a custom query.
- keep the node reference field as hidden field.
- While saving the node assign value(nid) in custom field to hidden node reference field.
Easier way will be overiding node_reference_autocomplete_value() in node_reference.module