What preflight passenger briefing items are required under Part 91?
As far as I can find, the only briefing requirements in part 91 are for large or turbine powered aircraft at 91.519 (which you assume exempted from), the one you found along with the seatbelt requirement at 91.107(a)(1), and 91.1035 which deals with fractional ownership aircraft. 

An AOPA article on briefings also states that:


  Operation of seat belts is the only FAA-required briefing item.


The only references to emergency exits under part 91 is 91.607 which pertains to passengers for hire. It only mentions the requirements for the number of exits and does not mention passenger briefing.