Try the app "ScreenDimmer" in cydia, it will allow you to set the screen to dim/turn off after a set amount of time without locking the device.
Edit: Just tried exactly what you want to do and it does work with ScreenDimmer. Set your auto lock to off, set ScreenDimmer to enabled, dim screen a short duration (30 seconds maybe), and turn off backlight to 5 or 10 seconds (that will totally turn the screen off 5 or 10 seconds after the screen dims). This will leave the device on and recording video, but the screen is totally off. Just remember to set auto lock to never.