I've attached a minimum org file that shows what I'm trying to do:
essentially to attach a hook to the export process that ends up calling
org-map-entries to make alterations to the file just before it's
exported.

As I've got it now, the hook does change the org buffer, but those
changes are *not* reflected in the exported output. I can only guess
that somehow the export process is able to continue on before the
org-map-entries results are returned? Doesn't seem likely, but...

This is actually kind of the opposite of what I'd like: to have the
changes *only* reflected in the export, and leave the original buffer
unchanged. On the other hand, I can see what thought and care has gone
into setting up a clone buffer for export (and I do need to rely on
buffer-local variables), so maybe an (undo) is my best solution for
returning the buffer to its pre-export state.

But I would like the changes to show up in the exported file!

Thanks in advance,

Eric