The general concern I have here is that this is making “system” create noise posts.
One alternative is using Discourse Policy to define a “policy” that applies to say TL0 in a tiny community, reminding them to post an “about me” topic.
I feel that getting System to do all the work here kind of detracts from the “Introduce yourself” aspect here where welcomes are far more personal.
There are a bunch of other options here, including unlocking a section of reviewable users (in our new reviewable system by @eviltrout) to a wider audience, so anyone say TL1 and up can “review” the new users and do the needful of saying hello and starting a conversation.
I totally get the underlying problem that, signing up is easy, making people say hello is hard.