I thought this might be useful to someone. When my dog chewed into and spilled a bottle of hydrogen peroxide on the floor with a 30 year old clear polyurethane finish, it completely removed it down to bare wood. I am not sure how long it sat on the floor but no more than 10 hours. I wonder if this would be useful in refinishing a small area maybe using vaseline to protect adjacent boards. It would bleach the wood I imagine, but sanding removes the patina also.
Tried it and did not have the same results you had. In fact I had no results. I tried drug store peroxide and then I tried 20 volume peroxide with the same results.
Lair, was your poly oil or water-based poly? If yours isn't oil, that might make the difference. Mimi's poly was 30 years old and thus almost certainly oil.