I'd say that running 180 degree water as underfloor radiant heat would probably be a bad idea. Most (all?) radiant systems use lower temperature water. Barefoot on a surface that exceeded 120 degrees would "cause the subject to display signs of discomfort" or some such understatement for a rather dry laboratory description of pain.
You might get away with it if the 'heat spreader' had a pretty good air gap under the subfloor. There would be no need to use a slant fin radiator since the sheetmetal would prevent convection through the radiator.