I still don't think an empty repo/non-revenue flight can ever be considered an operation under part 121. I can't find anything that says it can be, nor any examples or references of it happening that way.
In a somewhat related question to the FAA, someone asked for an LOI about whether experience in non 121 flights at 121 carriers can be used for the 1,000 hours of 121 SIC to upgrade.
Link Here's the answer:
"only those flight hours accrued during operations conducted under the operating rules of part 121 may be credited. This means that flight time accrued during operations conducted under the operating rules of part 91,
including repositioning flights and flight training conducted in a flight simulation training device are not considered operations under part 121 for purposes of satisfying the
§ 121.436(a)(3) experience prerequisite...."
Since this interpretation uses the exact same verbiage as the OE verbiage "operations under this part," I'm willing to bet the FAA would say repo flights can't be counted toward required OE time.
Now if someone can show me how an empty/non-rev repo flight can be operated under part 121, my mind might be changed. I've used all my google-fu tricks and can't find it.