Is it possible to SAML enable Sharepoint 2010 without ADFS or custom code?

I would like to integrate my IdP with Sharepoint 2010 using SP's native support for WS-Fed.  When I look at the SSO from ADFS to SP it seems like its just a SAML assertion wrapped in a WS-Trust wrapper; is there any reason I cannot simply mimic that behavior in my IdP?  While it seem like I could I cannot help but notice that no one seems to be doing this.  The two approaches seem to be a) use ADFS or b) use forms based auth and write some custom code.