Hmm... What page is that from? The reference I'm accustomed to is
http://www.avisynth.org/Tweak but it doesn't mention colorspace support.
The Avisynth version I'm currently using is 2.54 (January 26, 2004). Can you confirm that upgrading to the latest version will enable RGB in Tweak? (I tend to loathe upgrading my trusted software...)