This is now in place for trust level 1 — FEATURE: send message when a user reaches tl1 · discourse/discourse@41f76a7 · GitHub
I think it might be better to focus on the badges for tl2 and tl3. In broad strokes, my thinking is that your new users 
need the most encouragement; by the time people make tl2 and tl3 they are already super invested in the community.