I can't speak for Sitefinity, but I would be surprised if they told you "no you can't" if you wanted to build a custom web service interface into the data model. From a previous conversation, Sitefinity has informed me that they, in the future, may extend the API over web services. I have read multiple times Sitefinity advise not to build directly against the data model ~ consider this a sample, not something to use, only something to review as a possibility:http://www.konecny.com/SQL2008_552350_konecnyServices.asmx
This was generated using Codesmith directly against the database (BAD... Sitefinity will suggest not doing this). A much more elegant way would be to build a custom web service using the Sitefinity API directly, so permissions and content is channeled through its systems correctly.
Nonetheless, here is the source, just as a sample of how it might work, it is opperational, you just need to modify the connection string and add the web.config settings to your current web.config:http://www.konecny.com/webservices.zip
A good web service would be the perfect way to integrate a Windows application into Sitefinity (This is not that good web service, only an idea). I've disabled the actual functionality of the web service on my site to keep my database protected :).