1. You can cascode transistors to get higher voltage ratings. Mind that this gives voltage gain, so may tend to defeat the transition rate control, or introduce more chance for oscillation. So allow for the possibility of needing little RCs or ferrite beads around the active transistors, to control this.
You still need an aux supply to run the controller, but that can be a tiny little AC-DC module, or maybe you already have it from what's generating your 270V source.
2. These are aimed at hard switching, yeah; I don't see why they wouldn't be applicable to resonant as well, but those are quieter to begin with so it may just not be as important.
Resonant converters also tend to have a harder time avoiding diode noise (thinking of the LLC network for example), because the diodes conduct somewhat less than a half cycle each, that is, they kind of just do their own thing at whatever rate of current and voltage they happen to. (I have seen synchronous converters where the rect switches are operated at fixed duty cycle, which seemed to work out fine; controlled-transition could be used there to similar benefit.) And with the primary side switching noise being somewhat isolated by the series inductance, this can be a relatively large concern.
3. Probably just because making a controlled-transition driver and floating it up in the air, isn't easy to do.
Would be nice to have though. They make a version that's a controlled-rate driver with general purpose logic inputs I think, which would give an interesting opportunity to try. Both for higher voltages and for bootstrap drive if you're so inclined.
Tim