I love the examples, and have no objection to including this new element.

I have a question though: suppose I'd decided to treat the letter as a <text>
rather than a <div> (not on the face of it a completely daft idea), would the
postscript then become a <div type="ps"> or would it be inside <back> ?

Why not use <postscript> as the name of the tag? We generally do use fullnames
for these divTop/bot things (e.g. <epigraph> not <epigr>)