Let me suggest another solution since both 401k plans are managed by the same TPA:
Under Regulation 1.401(a)(31)-1, Q&A 14, Company A's 401k plan is required to return the improper rollover contribution to the participant, plus earnings. Let's assume the rollover contribution was $10,000, and there are now $1,000 in earnings, for a total of $11,000.
Under the EPCRS, Company B's 401k plan is required to attempt to recoup the overpayment (in this case, the improper rollover distribution of $10,000), adjusted for earnings (which may or may not be the same as the $1,000 in earnings noted in the prior bullet point).
Since we have the same TPA involved, couldn't the $11,000 in the participant's account in Plan A simply be moved back into his or her former account under Plan B? In that scenario, both plans have corrected the mistake as required by the IRS guidance. And I would treat this as "self-correct-able" under the EPCRS.
Thoughts? Anyone willing to poke serious holes in this proposed fix?