Use "$app" namespace identifier in app bridge resource picker filter/query?

It looks like it’s not possible to use “$app” in the query field (either the top level or the one in filters" for the resource picker, e.g. query: metafields.$app.my_field:MY_FIELD_VALUE. Seems like it ought to work if it’s using the graphql api under the hood but I’m finding it doesn’t. Is there a trick or is this just a missing feature?