If I look at a system as having three main components,
As *A* language, i.e. something to sit at layer 3 it makes perfect sense, some systems are best described in the functional model you propose, and it would work very well through the kinds of API that are in my proposal.
I can't see what we gain by putting RBML at layer 2, it gives a different way of describing the same compositing functions with no gain over existing VRML, and a substantial paradigm shift compared to what everyone has been building so far.
I also find it hard to see us requiring all behaviors to be written using the functional model, it is not ideally suited for event driven programming, nor for programming in a world of objects, events and Applets.
This obviously needs more discussion, but at the moment I would suggest that RBML be proposed for A language that is used to describe behaviors in a VRML2.0 environment rather than VRML2.0 itself.