If you're using WebSpace, just do it! Because WebSpace is based on Inventor,
it is run-time extensible. So, if you want to add a Voxel node to VRML, just
code it up using Inventor (see the Inventor Toolmaker for details on writing
new nodes) then put the code where WebSpace/Inventor can find it
(/usr/local/lib/Inventor/Voxel.so on SGI machines, for example).
Demonstrating working code is the best way of winning design arguments and
proving that something is useful and implementable.
If it is too hard for you to implement... well, then maybe its not realistic
to expect everybody writing VRML browsers to commit to implementing it, either
(which is what you're asking when you add anything to the spec).