Here's a couple updates to some of the nice posts you received thus far.
Fall fishing in Yellowstone means less anglers, usually.
Lake Lodge awesome place to stay
Never seen any shuttle services at Lake Lodge (Possible Lake Hotel but they could be closed by then
Boat Rentals end around first week September
You can fish the beach at Fishing Bridge and walk towards outlet
A marker will indicate your boundary (Used to be if you could see the bridge you went too far)
And you can also access it right from Lake Lodge Cabins.
Nice spot on the lake at Gull Pt near Marina Bay.
Less people at Sand Point
Last but never least - BEAR SPRAY a must.
And as one poster said, you might want to put more resources into a guided trip one day just to make sure you get what you came for.
Pick up a guide book for fishing Yellowstone at your local book store. I keep going back to the one written by Richard Parks of Parks Flyshop (Gardner,MT) but there are other titles as well.
Enjoy and we'll expect the full report.