Hey Ben,
Let me know if I'm understanding you correctly. You are trying to limit access to certain tab links for your portal users correct? The way you're doing it now is by limiting the roles that can view the tab links (Customize > Tabs > Edit Tab Links... > Roles That Can View) This will only work for actual Method Users that sign into your CRM, not Portal Users.
This may not be the best solution for your needs but one way of doing it would be to add a new dropdown field to your Contacts table (e.g. PortalRole) then create and assign the appropriate roles to your users that will be logging into the Portal. Next step is to hide your Portal tab links (Customize > Tabs > Edit Tab Links...) Finally, you will need to edit your portal Home Screen and for the OnScreenLoad event you want to show the appropriate tab links based on the users PortalRole.
There are other ways of doing this. Hope this helps!
- Ashur