Try looking at the actual Proposed Treas Regs that are use as guidance by most people in the industry:
http://www.irs.gov/pub/irs-utl/tres_reg-1125-2.pdfI think that Q&A 7 should satisfy your need for direct addressing that premium should not be reimbursed through an FSA.
You might also want to search the old threads.
The deduction for health insurance premium on a pre-tax basis, whether for COBRA or otherwise, is not done through an FSA but through a section 125 Premium only Plan (POP). Eligibility, or lack of, re the existing health plan should not be an issue since these employees will not be participating in that health plan. However, check the eligibility requirements of the section 125 Cafeteria Plan. The health plan deals with coverage while the section 125 plan deals with pre-tax deductions. These employees do not need coverage (they already have through COBRA) they need pre-tax deductibility.