I remember hearing from a friend at one animecon there was a guy dressed up as Godzilla and all his friends dressed up as buildings that Godzilla started smashing.
Maybe you should grab a whole bunch of people to dress up as yomas, start whacking all of them with your sword, and then turn around and start *glaring* at the security guards if they cause any trouble for you.
I can totally see me causing a scene like that too (unintentionally). No matter, if I get the right kind of plastic, it may still do the trick My hope though is that if I glare at them with enough "killing intent" then they wont try to take it from and all will be well.
When you mentioned that they do not like you bringing metal swords to cosplay conventions the first thing I thought was that you had brought one before and made them nervous with it.
Cosplay Security Wolf Briagde (CSWB) Commander: Sir, we do not allow metal swords at this convention. Put your weapon on the ground and step away from it!
Cyclone: The sword is the soul of a warrior. I will give you my soul only in exchange for yours *eyes glow*.
CSWB Commander: *backs up and draws gun* Get E-SWAT down here now! Look we do not want any trouble!
CSWB Lieutenant: But we just allowed those girls in with metal replica's!
CSWB Commander: Idiot! Their swords were not real! Look at this guy!
CSWB E-SWAT: Just put the sword down and no one has to get hurt!
Cyclone: You can have my sword... if you can pry it out of my cold dead fingers...
Narutard Sasuke: Wow! It actually looks like that guy with the sword is cutting off those guys arms while they try to shoot him! That's great!
Narutard Sakura: Is the sword guy on fire? How did he do that?
Narutard Sasuke: Man we have a long way to go...