Yes, I found a thread about "auto-changing users" somewhere here.
I wouldn't need an auto-change, I think connecting the limit to user-accounts would just make more sense. Forum admins set limits per user, so that's what I'd expect from a poster also
You already have support for proxies and proxy-per-user (or plainly: "multiple identities") so thinking about changing the way limits work would be just logical in my opinion. I'd never want to limit a forum in whole while posting, I'd just want to limit the posting user.
For my kind of application I'll code something on my side that spreads the posts among the accounts using post by directory since I am at a point where I really need this.
Wurstmu