overlay screen with .tga-file

[The chaser]

Never tried this, so maybe it's a really stupid idea, but could it instead be possible to just stick a billboard or a plane with the gasmask texture on it to the player? So that it would always be in front of the camera?

I don´t think this will work, because you have to refresh the texture with every step you walk. Anyway it´s not even possible to add own texures to Amnesia (at least I haven´t found a way).

I sent a pm to Palistov, hope he will reply...

I think Palistov doesn't reveal his secrets. Although you may try:

I've never tried it, but make a completely empty entity: Then, make a shape and then a body: Then, attach a billboard to the body.
Then, with script, Attach the model to the player.