I don't see why it wouldn't be allowed. The requirement for FPS being shown is just that, it has to be shown at all times. I would wait for a mod to chime in, but like I said I don't see why not.

If your purpose is to help with aiming by putting something in the center of the game then there's really no way to ban it, some monitors even have built in overlays that are undetectable unless you're pointing a camera to it.

screen centring is just using an in game mechanic to your advantage. Its not an issue