It'll probably be fine. You see this all the time on the auction site when a bow is being sold by people who have no clue.
I think I remember hearing one old timer say that's how they stored their recurves back in the day.
Another point, when I've seen this done the string is too long to really put a lot of tension on the bow anyway.
Guy
Edit: just re-read the original post and saw "longbow." In that case the bow is probably under some tension if it's strung backwards. I still think it will be ok, though.