After messing around with this and considering options, here are the available options:
A. Tweak a jpt! value screen effect so that it effectively gives you nightvision when you use your flashlight (the flashlight would trigger an effe which damages the player, applying the screen effect from the damage tag). See the MGM Weaponry Collaboration mod for ideas if you like. The problem with this is that you wouldn't be able to turn it off... although I do recall being able to overcome the effect by using the flashlight or by some other means that I have since forgotten.
B. You can do what you have been doing and have the background scope bitmaps be a different bitmap that has a black mask applied to it. You would need to use a rather large bitmap image, like 640x480 or something like that. Maybe modify the "ui\shell\bitmaps\trouble_brewing" bitmap to have just a black image and a black mask. That should do the trick... probably. The problem with this is that you still need to have a zoom in order to use the nightvision.
You already did something like this. The problem is that the Screen Effect is a special section of the wphi tag type and I don't see anything that -- without a great deal of effort (BUT YES IT IS POSSIBLE) -- you could do to get a similar effect in the hudg or unhi tags. But those would be the main experiments... if all the other options I suggest here do not work for you properly.
C. You could have an effe linked to the cyborg flashlight (in the bipd tag) which chainspawns something similar to a teleporter scenery model with its green glow in front of the player, so they look through it and what they see is lit up. Something like this:
where you have the teleporter shield model spawning with a garb tag in front of the player's view. Of course the ideal setup here would be just a new biped model that has this built into the model as a special shader, where it shows up when you toggle the flashlight. It could even be one-sided so it's only visible from the player's side of it. And if you just want it to work with a certain weapon, like having a Nightvision weapon, then you could try just swapping the fp model (ugh, nasty business) of the weapon you would like to turn into a nightvision wall.
D. There is some fancy-shmancy non-script-related tricks you could do involving the biped using the functions along with damage effects using jpt! and effe tags. But again this is less simple than just toggling off the zoom only bitmask flag.
Have you tried nulling out the flashlight attachment in the biped tag and trying again? Or messing around with the C in function in the biped tag?
Either you are groping for answers, or you are asking God and listening to Jesus.