I had a situation like this years ago. I told them to put the non-top hat participants' benefits in a qualified plan (where they belonged in the first place). In that case, it was easy--the qualified plan was a cash balance DB plan. But you could probably put them into a DC plan employer contribution account (may require amendment of the DC plan to reflect full vesting). Does that solve everything? No, but it did make sure the non-top hat employees' benefits complied with ERISA's trust requirements and the problem went away (no unhappy employee was going to report, in that case.)
The problem with a non-top hat top hat plan is that under ERISA, all employees who are non-top hat employees and who weren't in the plan have a cause of action to enforce benefits in a non-discriminatory way. In other words, if the top hat plan paid 5% of pay, all employees should have gotten 5% of pay (or some percent that complied with non-discrimination rules). Plus, the benefits in the plan should have been in an irrevocable trust.