I used PowerGrips for about a month, pooks. They feel very similar to clipless pedals when you're in them, so yeah, it's still the same issue. Clipless just holds the ball of the foot that much more securely than PowerGrips. The big difference I noticed with both PowerGrips and clipless was not so much in my feet, but in my legs. As for foot position, when the ball of your foot is all that's connected to the pedal, it's still easy to pull the heel up into a toe-down position.
You could try shoving your foot farther into the PowerGrip, but then you'd be pedaling from the arch rather than the ball of the foot and that's not as efficient. I'd think the best solution for this would be really stiff cycling shoes.