It is possible to use report models as a report data source from within Business Intelligence Development Studio (BIDS) although you have to jump through a few hoops to make it happen.
The connection string to use is:
server=<report server url>;datasource=<path to report model>;
e.g.
server=http://localhost/reportserver;datasource=/Report Models/Adventure Works DW;
Note that the connection string has two components that must be specified and neither component uses quotes around the values. You can then use the Report Model query designer.
This is all you need for SQL Server models but those using Analysis Services cubes require a bit of hacking because the Visual Studio version of the report model designer doesn't automatically specify a perspective and the query will fail without one. There isn't a user interface mechanism for doing it so you need to manually add a custom property to the query definition. The easiest way of doing this is to design a report using Report Builder and extracting the definition from the query that it generates. The bit to look out for in the query XML is:
<CustomProperties>
<CustomProperty Name="qd:PerspectiveID">
<Value xsi:type="xsd:string">
http://schemas.microsoft.com/sqlserver/2004/10/semanticmodeling/udmmodeling:
Perspective_Cube_Adventure_Works</Value>
</CustomProperty>
</CustomProperties>
Note that the <Value> field should not have any newlines in it and the query XML is actually embedded and not part of the RDL schema to the text above needs escaping (< becomes < etc). The best way to create these reports is with the report wizard as the query text is displayed on its own before being put into the RDL. Then you can merge the perspective without having to escape it.
Using this mechanism you can edit any report created by Report Builder. Just open the report in BIDS and replace the connection with a Report Model data source. One thing you can't easily do is round trip from reports created in BIDS to Report Builder because much of the metadata that Report Builder needs is missing. Still, it's useful to be able to create a large number of basic reports using the builder and then finish those that need it in BIDS.