Sorry for being late to the party.... September has really had me hopping at work being the first week of everyone getting back to work around here.
Have you thought to use rules?
I haven't investigated but it should be a rule or an action or even rules, actions with a helper module... I did something like this a few years ago, I'll see if I can find my notes.
I think it would require some experimentation to figure out if you set all registrations to require moderation and then allow those without the specific checkbox to get in automatically and send the hello message or whether you allow all to register without moderation and then use rules to identify that checkbox and hold them back if it it the specific value. I'm leaning towards the latter.... I think it would be easier to do and more fluid.
I'll see if I can dig up my notes... It was an experiment and not exactly what your after but I built a helper that worked with rules or actions, I don't recall which at the moment and it never went into production so I don't really recall.