Hello Morgan
I do unterstand that you want to offer your client's some special features and therefore you give only permission to the "hacks and mod" Forum of the Pro Version.
But to be honest, one of the decisions pro PRO version is surely the Plugin Systems. But without having the possibility to see if there are any interesting Plugins available it's quite strange to decide
So may I propose something to you: Handle it as the do with vbulletin.org: Let registered Users view ALL Forums, but restrict access to CODE and to Attachements. So only your customers can download Plugins and view hardcoded - e.g. for templates - modifications, but the potential customers can see what's "on the market".
cheers,
Roman