For 2 of the 3 weapon-confiscating casinos, yes.
I tested it out using the kill
console command on the three door greeters at The Tops, The Ultra-Luxe, and Gamorrah. After being killed, no other greeter took their place.
For both The Tops and The Ultra-Luxe, killing the doorman had no effect; meaning once they're dead you're free to enter the casino and gamble, etc.
With Gamorrah on the other hand, if the doorman dies the Omertas will immediately become hostile towards you, thus barring you from using the casino.
I also tested the difficulty of killing the greeters without using the console, and it's surprisingly a lot easier than I expected. Using a Stealth Boy and a hold out weapon, you can simply sneak up behind them and kill them without anyone else noticing. This can also be done at a relatively low level, thanks to the effectiveness of the stealth boy.