employee after tax contribution for 50% of missed def opportunity?

[Guest chrli]

Employer missed an employee's deferral election for bonus paid January 2011. Error was just discovered in Feb 2012.

The employer has asked if it can permit the employee to make up the remaining 50% (after employer makes the QNEC for the missed deferral opportunity) Tripodi mentions permitting the employee to make an after-tax contribution in this scenario. Has anyone ever done this?