There is no way to guarantee what you want. Granted, a /notice is the only chance you have, since a /msg goes to a separate window, but if the person in question has his mIRC set up or runs a script that diverts notices not to the active window, you can't make it go there. And if the channel you want the notice to go to isn't active, there is no way for you to direct the notice to it. It's all in the recipient's hands.
