I'm looking at getting a trainer because I have next to no time to get on the road and want to start up with Carmichael's Time Crunched Training Program. Part of the program is to ride do high intensity intervals at low RPM. I like the idea of a fluid trainer being the quietest due to training at night with a young one in the home. Also that I don't have to modify the resistance as I'm riding, but my concern is whether I can do high intensity training at low RPM on a fluid trainer. I've read that the resistance increases when the fluid thickens. And that the fluid thickens by the speed of the wheel in the chamber, so I assume that spinning at a high gear would allow for this, but I just want to ensure this is possible before ordering one.
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.
SuperMagneto Pro: http://blog.trainerroad.com/cycleops-supermagneto-pro-added/