7 - Mapping multiple LDAP groups to single Drupal roles using Boolean operators
Until I can find (or someone else posts) a better way to do this, my interim solution is:


Set up LDAP group -> Drupal role provisioning as normal.
Use the Rules module to assign/revoke another role programmatically on events User has logged in, and Before saving a user account.


Example (referring to my question above):

LDAP Provisioning:


Finance_dept|finance
Managers|managers


Create roles manually:


financemanagers


Rules:

Finance_dept + Managers = FinanceManagers


Events: User has logged in, Before saving a user account
Elements: User has role(s): finance, managers
Actions: Add user role: financemanagers


Removed from either Finance or Managers


Events: User has logged in, Before saving a user account
Elements: 

NOT User has role(s): finance
OR
NOT User has role(s): managers

Actions: Remove user role: financemanagers


Works for now, but it won't scale well beyond a handful of role combinations.