Can a report in PBIRS use a RESTful web service as a data source?

Take a look here - hope this helps:

https://docs.microsoft.com/en-us/sql/reporting-services/developer/rest-api?view=sql-server-2017 

Hello,

The PBI reports published to PBI RS should be able to get data from any REST API using OData or Web connectors.

Take a look on the following entries:

https://community.powerbi.com/t5/Integrations-with-Files-and/Using-a-REST-API-as-a-data-source/td-p/50400

https://community.powerbi.com/t5/Integrations-with-Files-and/Pull-data-from-a-REST-API-Authentication/td-p/246006

Let us know if this answer your question.

Regards,

Víctor M