You cannot accomplish "perfect FFA" without going into assembly. Don't attempt it in a plugin. You will encounter three problems, and only be able to solve at most two:
- Punishments, such as text/kicking/negative scoring.
- Damage reduction (it's 35%).
- Proper scoreboard display
If there was enough demand I'd move the ability from CSDM and into the cstrike module, but I don't think there is.
I might be able to make the CSDM module have "FFA without respawn," though.