This sounds like an interesting challenge! How about using PVC and making some sort of rail system. Put it over the railroad ties and well out into the beach/mud area. Move the boat onto it from the lawn and slide it down. To retrieve, an inexpensive boat winch (manual to save money) and crank it back up the same rail system? I'm envisioning maybe 2 inch PVC with 1/2 inch PVC spacers to hold it apart but with a rope run through the outside rails, through the spacer PVC and out the other side to keep the rails from collapsing. It would sort of look like a ladder when you were all done.
That is EXACTLY what I constructed, ad hoc, up at my in-laws lake front property. Their waterfront is a very jagged rocky slope - I used pool noodles over the "rungs" of the PVC ladder for slide-ability. It works, but it does distort from rectangular shape to more of an obtuse parallelogram shape. If you can anchor the top solidly it should take away some twist
I was working in caveman mode, ie, very little tools and the nearest hardware store being a mom&pop shop 40 mins away so I was quite limited (makes Home depot being 3 miles away from my home seem dreamy)