Just checked out 3148, and this issue has still not been addressed.
I see only two choices - either the '-- ' separator needs to be changed/deleted for all replies/forwards, or better would be if the separator was left alone for bottom-posting but changed/deleted when top-posting is selected.
This definitely needs to be addressed prior to the next update to stable. Top-posting users will complain loudly if the quoted messages that they forward or reply to are stripped below their reply. This will negate all the good work on making top-posting possible.