You could do what we do, and use a directory access program... Where you would put a .htaccess file in the forums dir... Then use the htaccess admin programs to add people through the browser! :)
Check out:
http://cgi.elitehost.com/acctman/We use this, and it rocks! We use it to make "premium" members to our forums. Then when it gets REALLY busy, you can close the forums off to everyone except those that paid for the extra service.
You could do the same sort of thing with this, and offer online access via registration to this program, for the amounts of times YOU specify! :)
Deejay