Just warn everyone that if you do NOT have a red marker, you WILL be shot and if you are shot, you cannot, under any circumstance; complain, bitch, moan, holler, etc.
If you do not want to get shot again, bring a red marker. Simple as that.
So to put this into a rule form:
If you do not have a red marker and you are down and you get shot again, you have absolutely no right to complain. If you are down with a red marker and are shot again, you have every right to complain.
This doesn't make it mandatory to have a marker, but it protects the people who do have them and entices the other people to carry them.
As far as the laying down after being shot thing goes...
If you are not in fire's way and you have a red marker, I see nothing wrong with sitting up with the red marker clearly visible. But.. if you are in the way, red marker or not, laying down or not, you will be shot again. The main thing is to just get out of the way of the game. If that means laying down right in the fire lane, so be it. If you have an opportunity to get just off the edge of the danger zone and sit up with a red marker, definitely do that.