special RADIUS hotspot accounts blocking URL

Hi,
I am using RADIUS accounts in my hotspot service since three years ago. All these account have a free navigation after a successfull log in.
However, my company wants to offer in the same hotspots a type of user who only connects to one specific URL. For instance, vocento1 user only can navigate in http://www.vocento.com/ . My old usernames and these new one have to autenticate in the same MT.

I read carefully the MT manual: http://www.mikrotik.com/testdocs/ros/2.9/ip/webproxy.php
Moreover, the MT articles:

I know I have to use this Mikrotik feature, however all accounts are RADIUS usernames with specific RADIUS attributes.
What RADIUS attribute do I have to use for assigning the web proxy feature? Is Mikrotik-Group RADIUS atttribute usefull for my goal (see http://www.mikrotik.com/testdocs/ros/2.9/guide/aaa_radius.php ) ?

How can I autenticate my normal usernames via RADIUS and the vocento usernames in the same MT device (but ,maybe, not in the same hotspot profile) ?

Is it possible to use the web proxy for several hotspot profiles?

Santiago