Rebol tries to get away with "everything is a BLOCK!". But having data structure interfaces that add more functionality to blocks... or remove functionality from blocks (if the operation would compromise the data structure rules)... is important.
BLOCK! is a bit of a too-simple foundation for "interesting" data structures. But although we now can have an arbitrary number of Extension Types, it doesn't seem like making people use a C compiler just to make something like a STACK! or QUEUE! (or the structure you describe here).
I've become more amenable to things like @rgchris's Node Proposal, where we'd offer a type that could serve as a foundation for more kinds of data structures.
But your non-expanding block could probably be done without Node, but just be a class of some type that wrapped blocks. We need to make it possible to make classes that hook into things like APPEND and such, and that's getting closer...