Heating pipes are not regulated by the plumbing codes. They are regulated by the Mechanical Code.

Unfortnately I don't see any 'mechanical' forums here. Has anybody seen this before??

I'm now getting quotes for adding water baseboard heat in our basement. I've got the stud walls up now. From these conversations it seems that it's ok as long as they're not in an uninsulated outside wall. And even then, one guy tells me it'll probably be fine since I had put a layer of 3/4 solid extruded foam on the foundation wall before I put up the stud walls.

If you want to read up, your local library probably has the code books in the reference section.

Unfortnately I don't see any 'mechanical' forums here.
Its called "Heating & Air Conditioning".

Here is a link that might be useful: Heating forum

While there may be certain construction methods that must be employed when installing hydronic heating pipes in walls, I don't know of any codes that would prohibit it. The shower in my bathroom has hydronic heat in the walls (including one that is a well-insulated exterior wall). I never have to step into a cold shower.

My next door neighbor's house (he's a retired professor who specialized in thermodynamics) has hydronic heat in the ceilings. His house is always perfectly comfortable, from floor to ceiling. Radiant heat will travel in any direction. Despite what most people think, heat doesn't rise (warm air rises), heat always moves from a warmer place to a colder place -- in any direction.