So what if you can speak as another player? I know there was something like this but it's not updated.
thing about that one is Makes <player> say <message>. not you say the message unless thats what fallout wants but idk.
I actually made a plugin for this for fun a while back, feel free to decompile it if you're paranoid. command fakesay <playername> <message> permissions: fakesay.say note: this only works for online players! click me if you'd like the source code feel free to decompile it, or click here
You could try /sudo [player name] [message] What this does is forces the person to run a command, and in this case it might go out as a chat message. Just an idea, not sure if it will work. Example: [me] /sudo Falloutmine spawn This would force you to run /spawn
that requires the essentials plugin, also to impersonate chat you do /sudo playername c:MESSAGE, as stated in the command reference