/tellcommand that you can use in the clients console.
In your plugin_admin.xml make sure you have the proper rights set for the admin to use the hidecmd setting.
Lets assume the private command for your game in this example is indeed
/tell and you want to warn the player for cussing.
First issue a
!list command to find your client slot number
Where the number 4 is your client slot number
You'll then send a private message to yourself with the command, so nobody else will see it, but it will get logged so B3 can process it.
/tell 4 !warn xlr8or cuss
The player will get a private message with the warning and nobody else will be able to see it.
On bfbc2 it's even simpler.
/ in front of your command and it will be private. So we can issue the warning like this:
/!warn xlr8or cuss