Pass parameters for node access on RESTful call
The entity index resources unfortunately cannot be used to filter by field values, they can only filter by entity property values (e.g. node nid, title, type, etc). I think you have two options:


create a custom service resource that uses parameters and a database query (or entity field query) to filter and return what you want
use views_json (a sub module of views_datasource) to create a View with a contextual filter for your field