51 year old retired from company. Received distribution from qualified pension plan, rolled into established IRA. Started substantially equal periodic payment schedule using the amortization method of early distributions from an IRA.

3-5 months later, 51 year old was asked to come back to company to work on special projects and has resumed a full-time position.

Does his re-hire at same company impair his early distribution exception using the substantially equal periodic payment schedule, thereby subjecting him to the 10% early distribution penalty?

I don't think so, since his money is in an IRA, which makes it a separate issue from the qualified plan. As long as the payments continue, the 10% penalty is avoided.