Block External Outlook Web Access only for few users

Hi,

We are using 4 Exchange 2010 Multi Role Servers with F5 Load balancer for OWA access from outside.
Right now OWA is accessible from Inside and outside environment for all users.
As per our client he have a requirement to "Disable the External OWA access for only few users and only during weekends" . There should be no impact to Internal OWA access for these users.
Let me explain the condition once again
1) Disable External OWA access
2) Only for few users
3) Only During Weekend and
4) No impact to Internal OWA access for these users

Is there any way to do it using Exchange 2010 or F5 or TMG and how feasible is that to deploy?

Thanks in advance for your help.