I'm gonna go out on a limb and say that this is probably hardcoded.
If you are whispering in a way that is fairly inaudible to anyone else, I generally emote: *leans over and whispers in X's ear* on talk, and then send a tell to X with the content of my whisper.
HOWEVER, if you do this you have to remember to emote everything that the other characters could see/hear, otherwise its abusing the game mechanics to send secret messages. Also, the other PC must be willing, otherwise you will not be able to whisper to them in a way so that no one else will hear because they wont let you get that close to them etc. You also have to be fair in saying that if there was another PC very close, that they would have heard it too, and send them a tell saying: "*you overhear them whispering* [content of whisper]"
There have been some instances of people abusing RPed tells in the past, some that I was the tellee, and some that I just heard of.. so, basically, know the people you're playing with, and only use this if it fits in with the group and you know everyone is cool with it and have a bit of trust between the players that such things will be handled fairly (if someone can overhear, you'll send what they overheard to them, and you'll emote whispering, because even if they cant hear you, they can tell that you're whispering.)
Never use this to harass another character (or player.) If the person you are trying to whisper to, is unwilling to engage in a whisper, you will be unable to whisper to them effectively.
-TV
