Give usergroup access to Paid Subscriptions page, but not rest of forum?

[WHO]

I have a pay only forum. How can I make it so someone who has registered an account (but hasn't paid yet) is able to access the subscriptions page.

Turning "Can See Forum" for the temporary usergroup (I'm using the default "Registered Users" group to hold the people who have registered, but haven't paid, and then having the subscription move them to the paid group) screws up my sign-up process.

As it is, someone will register for an account, be redirected to the forum home (where they'll see that there are 0 posts in every forum) page, and have no instructions on how to purchase a subscription. I want the standard error page (the one with the login box, which I have customized to give instructions on the registration process) to show up unless they have paid, so they won't be confused.

But if I do that, they can't even access the subscriptions page. That doesn't make any sense.

Is there anything I can do?

(Having the ability to pay when registering would solve all of this, I don't know why that's not an option)