how to query SQL Server via REST to get XML

Read here: Creating an OData API for StackOverflow including XML and JSON in 30 minutes. Basically, the road forward is for REST to be offered by app layer (WCF powering EF that provides the OData mapping). IMHO straight HTTP access into the engine was a very bad idea to start with, nobody liked the HTTPEndpoints of SQL Server 2005 and they were as misguided as it gets. One cannot map the HTTP error model, security, type system into SQL and expect a smooth interoperability. Having the HTTP layer live in a dedicated app pushes the responsibility of handling the HTTP ecosystem into a component specialized in that (WCF), and the logic of mapping the REST model to the DB model ina component specialized in that job (EF).

It sounds like you may be wedded to an MS stack but if you're not, you can use restSQL in a Java EE container (Tomcat, WebLogic, etc.) on top of MySQL or PostgreSQL. restSQL has a full HTTP API with JSON or XML encoding. It offers two twists: updatable composite views and hierarchical composite views. The framework is extensible to other databases and addition of SQL Server is in its supported evolution. Check out http://restsql.org.

Another option is something like Dreamfactory. They have a SOAP to REST solution that allows you to connect to any database or service. I have used their free hosted solution in the past for projects. They also have an open source solution available. The cool thing about the service is that they use Swagger 3.0 to create service definitions in a nice front-end solution so you can test and create new endpoints.

I have used the OpenAPI 3.0 definitions to connect to 3rd party SOAP and REST services as well. They also support stored procedures and server-side scripting in the SQL Server environments.

Anyways, might be another option for you.