I looked and I'm surprised that I haven't seen any discussion on Sack Over the Head and the character's MPs, neither here nor anywhere.Sack Over the Head
Minion Permanent-event
Playable on a Troll minion whose company is attacking a hero company with at least one Dwarf or Hobbit. For each successful strike against a Dwarf or Hobbit, the character is not wounded, but rather placed “off to the side” with this card. Discard if no characters are placed with this card. Target Troll cannot move to a Darkhaven [ ]. At the start of your turn, make a roll (draw a #) for each character placed with this card. If the result is greater than 8, the character is eliminated. If target Troll is wounded or removed from active play, discard this card and the surviving Dwarves and Hobbits form a company at the Troll’s current site or new site.
Sack Over the Head has an effect which places an opponent's characters off-to-the-side with itself. It seems thematically similar to a "prisoner" card, maybe something like the once-mentioned-but-never-implemented "capture" mechanic. But the card doesn't say prisoner and the character is not a "prisoner," and the mechanics don't exactly line up anyway (being a resource). So then the usual off-to-the-side rules apply: "Unless stated otherwise on its host card, a card placed off to the side will give its marshalling points to its owner." The opponent is still the owner even though the character is not even in their play area (and the same could be said of many situations in which cards are placed off-to-the-side anyway). This seems novel but I guess the character would be worth MP if they were not eliminated by the skipped body check, and they will not be worth MP is eliminated by Sack.
Use Your Legs is similar and has a clarification about not being prisoners (as far as negative MP), but not about off-to-the-side positive MP: "The characters placed off to the side are not considered prisoners for MP purposes."