Ok, I've got an idea how it should be hosted;

The main AB server (which would likely be plus only) should be hosted on an alternate port. This means whenever plusless users try to join, they will get redirected to a general help server, hosted on a port that works for vanilla too. I'm not sure if it still works if the said server they try to join has plus only mode on, but I know redirections happen.

The help server would have a level that says something like "You may have been redirected. To join the bash you must have JJ2+. Download here: (link)". I'm not too fond of the camel help server level, since it's a bit too vague.