First off, this is *not* a finished helmet that I would propose to use in the game as it is - this is just a start, and at this point mostly to show a texturing technique I learned.
I struggled for a long time trying to find a painless way to make a helmet for the new soldier models, one that could use faces from the existing heads, without having to labor long and hard on multiple versions for each head, rebuilding them based off of the existing heads.
The procedure is now simple - I build just one helmet model, and for the "face" part of the helmet I use a custom texture inside blender that doesn't generate its own part of the skin by itself, but instead grabs pixels from a simple 2D plane mesh in front of the helmet. The 2D plane has a front head-on shot of the head, taken with an orthographic camera in another Blender scene.
Once the custom texture is prepared and the plane aligned properly, it can then all be baked into a regular skin and exported with the helmet as an MD2.
For different faces, all that is needed is to swap out the 2D plane and tell the custom texture to use a different mug-shot of a different face.
Virtually no mess, no fuss, once the initial helmet is made and everything is set up.
EDIT: Darn, the image upload folder is full again... Hang on a few minutes...