Hi Adam, Lee here. If you're going to make more than a few image filter effects for CamTwist, you should probably be informed about the Render In Image patch, so I thought I'd try to assist. (But if the Graphic Animation Protocol works instead, cool! That's that, then.)
Regarding Image Filter Protocol in general however; you can usually overcome the consumer-at-root problem by placing the Billboard inside a Render In Image patch. That's the standard work-around for when Billboards/Sprites are needed at root in protocol mode. Just attach the protocol Image (Required) patch to the output port on the Render In Image. At a minumum, you'll also need to publish the Billboard's image input so it will appear as an input on the Render In Image to be attached to another protocol Image (Required) patch. Other Billboard input ports can be published as well, or they can simply be addressed from within the Render In Image, if desired.
Intel Core i3, ATI Radeon HD 4670, OS X 10.7