I do the same thing, however, I use two mods for this
Post Count Requirements
- set every forum except the introductions forum to require 1 post
ACP Announcement Centre
- put here that's it's required you post in the introductions FIRST.
it's not as good as when they go to another forum and getting the message they have to introduce themselves when clicking new topic, but this is the only way I can think of that might help you a bit.