Trainers: Fluid or Mag
Using bikecalculator.com, I have a 165lb rider on a 15% grade at 12kph (units, oy!) needing roughly 500W. I'm pretty sure that most decent trainers can generate that, including the SuperMagneto Pro that I use. (My power meter and trainer road have told me that I've hit those numbers, however briefly). Like you've indicated, the progressive resistance is generated by speed, so you need to use an appropriate gear combination (big/small) that lets you pedal slow, but spin the wheel at a greater multiple of pedal stroke.

Power Curves:

RoadMachine: http://www.kurtkinetic.com/powercurve.php

SuperMagneto Pro: http://blog.trainerroad.com/cycleops-supermagneto-pro-added/