Well, again, I think it would depend on the design. The radiant heating discussed in Craig Hopkins' article, which operates by heated water passing through pipes under the floor, works really well, never gets too hot, and is really safe, as far as I know.
I think there are other systems that work similarly with electric cable laid under the floor, with tile (I dunno what else, would have to check) on top. I think those are pretty safe too. The way they are designed, they never get too hot to walk on with bare human feet.
But lots of things are called "radiant heaters" -- including half the space heaters sold at WalMart -- and those small, not attached to anything, stick up in the air and make lots of heat heaters would be incredibly dangerous in a coop or barn, seems like.
Can you show us what you bought for the pigs?