Edit: Also, as I think about it, one could maybe argue that closures are to expressions as blocks are to statements. In which case, it seems that blocks should never be bound to values but things that invoke blocks should only be able to do so as in the Ruby 'yield' where the block is implicit. You could then avoid the whole escaping problem.
Edit: And further, a thing which invokes a block should maybe belong to yet another category from closures and blocks? In a sense, such a thing is analogous to: