Good news! It works!
Bad news! It gets really hot!
At the suggestion of tech support, I connected the oscillator pin to the gate to select a "constant off time" mode as referenced in app note AN-50. This allows duty cycles >50% without subharmonic frequencies appearing.
Now, it's putting out the rated 1.4A, but everything is getting really hot! The inductor and diode gets warm (to be expected), the FET gets hot, and the HV9910 gets hot! When I say hot, I mean 150+F. I put the fet on a heatsink, but not sure what to do with the 9910 - or even why it's getting hot.