As you probably know, the custom rendering specification wasn't documented with RS 2000. The features set was there, only the information was lacking. The SQL Server 2005 Beta 2 documentation now includes the rendering extension specification.

James Yip has written an excellent article about writing custom rendering extensions in the February issue of the MSDN magazine. As you can imagine, custom renderers are not trivial to write. For this reason, I highly recommend you check for availability of third-party rendering extensions that support the rendering format you need before you embark on this journey. For example, as I mention in one of previous posts, SoftArtisians provides flexible Excel and Word rendering extensions.

Happy rendering!