Custom FormsAuthentication by means of a proxy...

I need to understand how to go about and implementing a custom SSO in SSRS. The scenario is that I have a custom J2EE application that handles authentication and I need to make SSRS to use it. I've done quite a bit of reading but is still unclear about possible ways to do it.

One way could be to place a proxy that uses webservices (SOAP) to call on RS. When user accesses the page, it would check the session for the SR token and if it is not present it would check with the authentication server if the user is logged on and if he is, it would log him on to the RS and store the generated token in his session. Otherwise it would simply pass on the token to the RS. Is this the correct way of thinking? Are there any other possible ways to do it? Would it be possible to generate a correct cookie from SOAP-reply and set it for the user so that he then could automagically access the RS withouth the proxy?

As I understand, one could also implement IAthentication (?) interface (the one with the logon-method) that could call on the authenticationserver. This would enable direct display of the reports without a proxy, but the code would have to be written in .NET (so far I failed to find a single code-sample that would work in a fresh install of development tools for RS2005 and that includes msdn). Any comments on that?

Thanks in advance!
Nobody knows anything?