Launchpad authentication is only available on AU. If you use that to create an AU profile, would doing so create a profile for you on the rest of the network?
If you sign up to Ask Ubuntu via Launchpad, would a profile get created on the wider SE network?
Yes, it should.
However, you cannot use your Launchpad OAuth login on any site other than Ask Ubuntu. So you should also add either a StackExchange local login or a secondary login from, say, Google or Facebook, in order to authenticate to other sites (when cross-site cookies don't work or such).