Most of the time I can back my trailer completely underwater and float the boat right on and off, completely rigged.
It helps to have low profile tires on the trailer, and an extra long tongue. My trailer also has a tilt bed, which is handy when you have a steep drop. Having a good 4x4 is helpful too for primitive ramps, with high potential to get stuck.
When I have to winch, I connect to my grab line, which runs free through my D-rings so as to evenly distributing the weight to all the d-rings. I usually don't run a super heavy boat, so it's not pulling too hard.
I like the idea of the sling underneath connecting to the oarlocks though.