Hi Blink, I think you need to create the entity mirrorsprite first,
From the manual "I hope it suits A6" you = ent_create("flash.pcx", temp, flash_action).
Have a look in the manual for this under Engine Functions/Entity Functions for more info.

Then have a look at the bmap_for_entity function. I am sure you will need a wait(1) right after you create the entity to give the engine time for creation.
You may also need the proc_late as well.
If you can give me a day or two I might still have my old A6 verticle mirror code lying around.