Can we call external functions (such as Amazon Location Service functions) in Neptune?

The closest related feature that exist in Neptune today is the integration with Elasticsearch/OpenSearch, which allows users to perform a full-text search query against an external OpenSearch index. https://docs.aws.amazon.com/neptune/latest/userguide/full-text-search.html

For RDF workloads, the SPARQL query language offers a federated query feature (using the SERVICE clause) to query an external SPARQL endpoint and return a set of triples/quads that can then be used as part of the query against Neptune. Instead of making a call to an external SPARQL endpoint, this could also be used against a REST API such that the API returned the results in the proper format. https://docs.aws.amazon.com/neptune/latest/userguide/sparql-service.html

Other than that, we don't have the ability to call other external APIs/functions from any of the other query languages supported inside of Neptune.