This task has been outstanding for a while, but I've managed to solve it in a very simple way using CSS columns. The solution is specialized to lists at the moment, but it should work identically for any other element on which we want to implement it. Notes/limitations:

• The implementation is triggered by the list having a child::cb, but it uses descendant::cb for counting the number of columns, on the assumption that some column breaks may occur within list items. It would fail to work if there were no <cb> that was a direct child of the <list>, though. If we were implementing a more general solution, we would need to figure out what level the columnar layout should be implemented on, and we'd probably have to require @type="columnar" or something like that on a block-level element, to trigger the appropriate CSS.
• The CSS still has to have -moz- and -webkit- prefixes.