It occurs to me that this is the same mirroring needed for toner
transfer.. Do you have to do similar?
I just did a board[*] that uses toner transfer to etch the solder side
of a board I designed from the top. It took a few minutes for me to
figure out whether I wanted it mirrored or not when I printed it. The
answer was "not". What I end up doing is disabling "auto mirror" and
exporting twice, once for each mirroring type. Then I have lots of
pages to choose from.
[*] http://www.delorie.com/pcb/r8c-1b-adapter/