Would it be possible to create a feature that allows the player to crawl on their belly?

Change the crouch animations so when you crouch you crawl

No like Call of Duty if that's what your thinking. You could only change the crouch animation to him crawling but it wouldn't look good.

Mmm... you COULD play a custom animation with the player when he presses the flashlight key... but then there are a few problems.

1. You can't move, it's more like hiding for your life.

2. Your view doesn't shift down. At least I don't think it does.

Just an elaborite idea...
Make him get in an invisible vehicle. The animation for getting in the vehicle is to get down in prone position, then, once in the vehicle, when you move, the player has animations (like turning his head when he's in other vehicles, but he moves his whole body to look like he's crawling). The vehicle has no weapons, and you use you're own gun and are in first person(like behing in the side seat of the tank, but you can drive it). Anytime a player presses crouch, but is already crouched, the vehicle spawns and they get in. The vehicle can have custom movement sounds that sound like someone crawling (don't know what that sounds like exactly), and is very slow, and has no geometry, so the player is the only thing that takes damage.
Or maybe there is some sort of "player state" tag that you can make and edit.