Does anyone know of any restrictions on 401(k) plans that cover only collectively-bargained employees from adopting a new comparability profit sharing formula?

I know some collectively bargained plans have radically different contribution rates for apprentices and journeymen.

Technically collectively bargained plans get a 401(a)(4) pass so that you could have any benefit structure you wanted and not have to do any testing. It would appear that you could give all highly compensated collectively bargained employees 100% of compensation up to $40,000 and give NHCE collectively bargained employees 1% and still pass. However, you had better make sure that you have a legitimate collective bargaining agreement and that all of the employees are "collectively bargained".