I've searched the forum archives for mention of this, and
got several hits. The comment is usually to the effect of:
"this is not yet supported, in part because the spec has
not yet stabilized". Most such comments are from one or
more years ago. So: any progress on this front, since then?
It will be cool, when and if.

I've added this feature to the roadmap, but it requires some major revisions to our layout engine and will need to wait until after the release of Prince 7.0.

Just a "me too"... I'd really like to see this implemented.

Prince 8.1 is now available, and supports "column-span: all" to enable a block to span across all the available columns. You can also span a specified number of columns using column floats.