In short, there's a reason that most forums have an automatic type of welcome thread created. Even SaviourPC, another forum which is MUCH less active them CH right now, and at the current time new members are only trickling in, has it setup to automatically create a welcome thread and also an automatic PM is sent to that member, with a link to that thread. I personally think this is a good compromise between added work and personalization, the PM has a different link, and so gives the appearance of being more personalized.
Right now, it would be quite possible to welcome each new member individually. However, as more members (we hope) start joining, it will become more difficult for moderators/administrators to sort out exactly who has been welcomed and who hasn't (a problem which has already cropped up here). Although it's feasible to create a mod where moderators are dealt welcome responsibilities in a round robin style, the monotony of welcoming various people whose aptitudes and ineptitudes one can only guess becomes a bore, and they end up writing essentially the same thing over and over again.
So it's feasible, but it isn't a scalable solution, it follows Parker's law far too closely. (work will expand to fill the time to finish it)
I'm not saying it's a bad idea. Ok, so that's sort of what I'm saying, but it's a thoughtful effort, that's for sure.