Having given this a little more thought, what I'm really looking at when I slide off the aft end of the seat and find it more comfortable is the difference between rotating the hip on the saddle, which puts the long parts of the hip (the bony parts going forward and inboard from the sit parts) contacting the seat and distributing the weight more, and sitting up in a typical comfort bike position, which flattens the butt, and straightens the back, ironically making the spine more in compression.
Next I'm going to try an extended stem.