How to properly drive MOSFET in MPP tracking duty cycle above 50% for a synchronous buck regulator at 50-100V input?
It's odd what you say about the bootstrap circuit not performing well above D=0.5. I'm using the same driver chip for a hefty 200 watt power supply where the duty range is quite wide and I've never seen this happening. 

I'm switching at 100 kHz - maybe you are using a switch frequency that is too low? Too low a frequency will cause bootstrap power to be a problem.

Another alternative is to use something like a little 2 watt Traco-Power or XP or Murata mini-power module to generate a floating 12 volt for your top FET.