Is it the dropouts, stays, or bond between them that is bent? whichever it is I don't think it will end well if you try and straighten it, but if it's only for trainer use surely you could just squeeze a wheel in there and pedal away? no need to actually straighten the thing, as long as the pedals and wheel turn freely, on a trainer it's not really a problem if the wheel isn't aligned or tracking in a straight line.