I installed JACLPlus so I could add additional levels of permissions to my joomla users.

In particular, I am using SubscriptionPro+JACLPlus to create a "Premium" level for those who donate to our site.

When someone donates, SubPro+JACL changes there group assignment from the standard joomla "Registered" to "Premium"

I'm just wondering how I could also set up the phpBB forum to have a section that would only be visible to "Premium" members.

Would this take a custom hack?

You can setup a Forum to be Private or Private [Hidden] ... the hidden version will not allow anyone not specified to view the forum or its posts... The other version will allow public to view but not post... Then setup a "closed" user group with yourself or some other admin as the moderator... Allow that group access to read the private forum from the group permissions screen... then from the front-end that moderator can add users to their usergroup...

Greg