To close this out, FWIW, I value bet $3 on the river more or less convinced that I had the best hand with no fear that anyone would re-raise me on a bluff. I wanted as many calls as possible and I felt 20% of the pot was about right.

MP and SB called. UTG folded. SB showed 5xxx and MP showed (get this!) T7xx (no straight or flush draws and no overpairs).

Obviously, I ran into some very big donks. I posted this hand mostly to see how anyone else would have played in this somewhat bizarre situation.