Change access level at runtime/XDS AX 2012

Requirement:

Per user define access lever to Customer accounts by customer group.

Example:

User01 has read access to CustGroup A & B, updateaccess to groups C & D and no access to groups E & F.   A new table stores the association between UserId, CustGroupId, and AccessLevel.

I understand how the security policy can prevent the user form CRUD for groups E & F.  I don't see a mechanism for changing/assigning access level.

Thanks for your help and/or advice!