The foam is rated at 25 psi as I recall. The concrete is closer to 3000, so it will distribute the load. But even at 25 psi, a 10" square baseplate will support 2500 pounds directly on the foam. Of course, you wouldn't do that--the foam would eventually creep if nothing else. But the stuff is easy to puncture, but really surprisingly strong with a distributed load.
If you lose track of where your tubing is, wait for the floor to fully cool. Mark off the areas where you want the bolts, giving yourself some adjustment room but keeping the investigation area as small as possible otherwise. Find your laser thermometer and make sure the battery is good. Find your marking crayon and have it in hand. Once you are ready, crank up the hydronic system to full tilt by raising the set point to 10 degrees above the thermostat reading. Stand at your spot, scanning the area with the laser thermometer at right angles to the tubing orientation (which you should know from the photo I hope you have from before the pour). You'll start to see peaks in temperatures forming. Mark those with the crayon. Connect the dots and you'll know where your tubes are.
An HVAC mechanical engineer I know uses a thermal sensing camera to do this.
Eventually, the floor temperature will even out and you'll lose the ability to see the peaks above the tube locations, so don't dawdle. Remember to turn the thermostat back down when you are done.
All that said, I think I'd prefer a four-poster if I had to have a lift, just because i've seen too many close calls with grossly unbalanced loads, or loads that become unbalanced as some heavy part (like the engine/transmission combo) is removed or added.
Rick "who may be facing the same issue someday" Denney
