From the point of view of recordkeeping, treating the amounts posted in the system as pre-tax and accounting for the correction as if an in-plan Roth rollover occurred is creative and gets the plan accounting close to what should have happened, but consider some of the other potential implications of of this approach.
The impact on the personal taxes for each individual could vary significantly. The amount of Roth deferral reported as taxable on a W-2 could increase an individual's marginal tax rate for the year for which the income is reported which would result in the individual overpaying taxes had the error not occurred. The amount of Roth deferral not reported as taxable could decrease and individual's marginal tax rate resulting in a larger amount of unpaid taxes.
If a correction happens to involve a plan fiduciary, company executive or HCE and the correction resulted in less taxes than should have been paid, then that individual's correction and the plan would be looked upon unfavorably by the regulators. This could expose the individuals involved and the plan to more serious issues tied to fiduciary responsibility, to nondiscrimination or to tax avoidance.
Conceivably, the plan may want to try to characterize the correction as an Eligible Inadvertent Failure. However, the EIF rules generally are designed to encourage self-correction, but they are not designed to allow a plan to make up its own correction method. Given that the IRS has a prescribed correction, it makes sense to follow it.
A mistake happened. Own it, follow the rules, fix it, take steps to prevent it from happening again, and know steps were taken to protect the plan and the individual's involved.