New Heated Garage: need help!
I personally don't think the heated floor is a good idea for a situation like this. It's no "quick" heat solution by any means, and I would definately heat the entire floor, unless this heated area is walled off. One other problem is that you really should have some kind of insulation vertically down to frost line, which isn't easy with a pole barn.
My shop has heated floor in 60% of the floor. I didn't even turn it on this last winter, due to the cost to operate. From my experience, you can't install one of these systems w/o some very knowldegeable, professional experience and expect good results.
If you still want to go ahead with the tubing now though, you could rent the stapler to fasten the tubing directly to the 25 psi foamboard. As a matter of fact, we rarely pour over radiant tubing that's tied to mesh anymore.