Can an employer allow a participant to have a Self-Directed account "frozen" to any new money, and not allow the rest of the employees to have one? Any cite?

The self-directed account is an investment option which is subject to benefits, rights, and features testing. If it passes on the date that you freeze the feature to new money, then it will be treated as passing thereafter. See Treas. Reg. 1.401(a)(4)-4(B)(3).

In summary, yes if the participant in question is not current a highly compensated employee.