I just opened the brain editor, under basic Responses, greetings, there is hello1 and hello2, from the content I'd say hello1 is dependent on the time, hello2 is not time dependent so I'd put new responses there unless... Hal already has a long list of responses for hello so it's unlikely youre response would be choosen.
I just started up Hal, under the options there is the startup, there is a say greeting during update where you can type something in. I'm not to sure how this would work.
Carl2