I have not seen a modification yet for your setup, but a few modifications do exist to integrate with external authentication systems such as LDAP,
Facebook,
Google. If i recall correctly these modifications integrate using the vBulletin Plugin system without the need to edit the vBulletin source codes, making upgrades easier as you will not have to re-edit the source with an upgrade.
Once you have obtained a license you can post a request for coders to take on the job if you are not comfortable writing your own modifications.