Is it possible to use the DomainDataSource control with a plain DomainService? I want to page/sort non-entity data.
Yes, if you build your DomainService, it will become available as a DomainContext. then you can use it inside your DomainDataSource.DomainContext property. A simple example from MSDN
<riaControls:DomainDataSource Name="source" AutoLoad="True" QueryName="GetProducts">
<riaControls:DomainDataSource.DomainContext>
<domain:ProductDomainContext />
</riaControls:DomainDataSource.DomainContext>
</riaControls:DomainDataSource>