Union Employees with Non-Union Wages

Maybe this is a simple question, but I'm having trouble wrapping my head around it and I've never had it asked before.

I have a potentially new client who wants a 401(k) plan. They have union employees covered by a collective bargaining agreement. So the premise would be to exclude union employees from the plan, right? Except they also pay these union employees non-union income to cover their travel expenses.  AND, one of the two owners works for the union as well.

Part of me says, well that part of the income isn't covered under the collective bargaining agreement (or able to be deferred, matched, etc), so that income would automatically be included, and they would be eligible for the non-union plan for the income not covered in the agreement. But the (prototype) language that any employee covered by a collective bargaining agreement is excluded has me wondering.  

Your thoughts?  (Thanks so much in advance!)

The EOB has an explanation of "dual status" in the coverage section.  It sounds as though it addresses hours of service in a non-union capacity specifically and the pay associated with those.