I thought the Trust Levels were more about learning the rules and mores of the individual forums. Allowing people to skip the two week/15 visit ‘acclimatising’ phase (default) based on other forums rules may not work out so well.
I wouldn’t mind an extra ‘associate member’ trust level that could be applied to forums/users registered to a central directory-hub. Something that would allow me read access to lots of forums, and maybe the option to vote in polls, or Like things - while still needing to sign up to a particular forum for full membership to post and progress in Trust Level, etc.