Multiple Billing Invoice Number formats within a business unit

We want to create unique billing invoice numbers within the same business unit.  The current format allows for one or two 'static' sections and one auto numbered section.  We want to be able to vary the 2nd static section to allow for a different value when billing by various departments within the same business unit.

Current format by Business Unit:   Campus designation - Next  = WOR_00000015

Want something like   Campus designation - Dept designation - Next = W_XXX_0000015

Any solutions or ideas short of printing a value on the invoice form (prefix or suffix)? I need the dept designation in the invoice number.