jam,
It sounds like you have the traffic coming in, you just have to get them to sign up?
I guess what I would ask is what incentive do they have to join? On our forum, we have some picture forums guests can't enter although they can physically see. And we also have our board set up so that guests can't view attachments, seach the member list etc.
Once you put together a list of perks for joining, use them as incentive for joining. And use the guest message in this hack to sell your guests on the merits of becoming a member.