Cheers, buddy! Unfortunately, unlike its predecessor BF2, Battlefield 2142 doesn't seem to support shaders, so the only way to make the background was to add it as an image overlay of the screen. Luckily, I got it to work with a few blending effects and it turned out pretty nice. Good luck with your project, I'd try coming up with some helpful advice but, unfortunately, I'm not very familiar with Doom.
I can probably do the image overlay stuffs just fine, though I am not an expert on shaders for the source port, GZDoom, which is the platform I am modding with. But the real problem is how will those boxes appear and resize according to monster height. I heard it is possible to do this on ZScript (a coding platform along with DECORATE), but since I am not an expert to it, might as well pass. So for my basis on the Smartgun's HUD on my mod, I'll use A:CM's, though it is less accurate to the base material.
Speaking of, the Smartguns tracks and detects infrared targets, but since the xenomorphs are not that type of targets you'd be seeing on the HUD, or notably tracking. You can't track an alien this way. A mistake that many games that feature both the alien and the smartgun (AvP, A:CM, etc.). So hence, for my mod (since it takes place years after Aliens), the Smartgun there has updated tracking mechanics and sensors inherited from the Sentry turrets (which does detect aliens and fires upon them) and the Motion Tracker. I used that as an excuse to also update the Smartgun's HUD to A:CM's, though in a much bare bones feature.
For reference, here is how A:CM's Smartgun HUD looks like.
Besides that, I wonder if there are any cosmetic changes to W-Y's weaponry, since in your mod, they use the firepower from the Marines. Like, the Pulse Rifle being black instead of brown bess or olive green akin to the commandos' pulse rifle in A3. That kind of stuff maybe.