We know that companies with only one director (and no other officers/employees) are outside auto enrolment (see s90 Pensions Act 2008).
However, for clients with more than one officer, which will be quite common where a separate person is company secretary, then the above exemption won't apply and we need to consider if the officers have an employment contract (and so fall to be 'workers').
Where there is no written contract, which will be very common, we still need to consider if there is an implied one.
Where the director is being paid via payroll, then we would have to argue the payments are 'directors fees' (i.e. for services provided to company to fulfil role of director) and not reumneration for other work done (otherwise there must be an implied contract for that work). The same applies for company secretaries.
Where payroll is the typical £10k/£10.6k, then the above would seem feasible. For higher payrolls there could be an issue.
I don't expect the Pension Regulator to take the above point in practice, but I suppose it is possible they could?