Thinning the Multi-Select Text choice pool

I've seen similar posts on the boards, but not one worded quite like this... :

Is it possible to utilize several MST fields to thin out an available options in a "master" formula-MST?

Imagine the following scenario...

A small "New Hire" app is requested by HR to easily initiate a mountain-sized pile of  tasks to be done by filling out a few fields in a form and pressing save, but can't be bothered to keep a list of active company extensions anywhere. The app is to be fed a list of  company contacts from another table.  The HR rep wants to only be able to select available available internal company extensions to avoid having to reprint Business Cards.  

Master MST pool : {401-449}
Department Summary Pools: 
Billing: {410-429}
Sales: {401-409}
Repair: {430-439}

Already-assigned Extensions include : 409, 412, 431, 433, 402, 413, 417, 404, 401

Our derived summary Pools then look like this:
sales_pool   :  401,404,402,409
billing_pool : 412,413,417
repair_pool : 431, 433

Is there a native method to remove the items in the summary pools from the master MST?  I already plan to break up the comparison based on their departments (for less computing).  What can I do natively to accomplish a list of selectable,  extensions  still available in the repair pool?

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Doug Hines | dhines@tritonnet.com
Learning it As I Go | Tier II Support | Systems
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