You can consume your data service in code and extend the Standalone Designer tool to use it e.g. as in the Connecting Telerik Reporting to OData feeds
blog post. In code you can implement authorization or other security feature to prevent users from retrieving specific data.
Other approach I can suggest you is to provide users with the data model and hard-coded sample data just for the purpose of previewing their reports, and on preview in the actual application to use other assembly connecting to your service. For more details about to changing the used data check the Deploying trdx (XML report definition) that uses external assembly
(shows how the actual application can be configured to use other assembly than the client's one) and Changing the connection string dynamically according to runtime data
(illustrates an approach for accessing data items DataSource properties at run-time).
About saving the report, the Standalone Designer produces a TRDX file and the save operation cannot be changed to work with a custom database. Thus you need to implement logic for uploading the TRDX file on the server, or reading and saving its XML.
Let us know if you have any further questions.
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