For hanging radiant surfaces, it appears that you will have to create BuildingSurface objects assigned to each zone where the radiant system is operating. From the documentation, "Note that all of the surfaces within a single list must be a part of the same zone and that the zone of these surfaces must also match the zone the radiant system is attempting to condition.Only base surfaces (walls, roofs, floors) are valid. Window/Door surfaces and Internal Mass are not valid surface types for embedded radiant systems". This is the procedure that I have tried with success:

1. Draw a hanging surface inside of the zone where radiant system is operating
2. Assign that surface to be a ceiling with a zone outside boundary condition using the same zone
3. Add that surface to the Radiant Surface Group Name object where you list all radiant surfaces in the zone
4. Assign that Radiant Surface Group Name to the appropriate ZoneHVAC:LowTemperatureRadiant object you are using

For Zone A, an example of this arrangement in your IDF file can be found below.

ZoneHVAC:LowTemperatureRadiant:VariableFlow,
    Zone A Radiant Loop,         !- Name
    RADIANTSYSAVAILSCHED,    !- Availability Schedule Name
    Zone A,                              !- Zone Name
    Zone A Radiant Surfaces,    !- Surface Name or Radiant Surface Group Name
   ...

ZoneHVAC:LowTemperatureRadiant:SurfaceGroup,
   Zone A Radiant Surfaces,              ! - Radiant surface group name
   Zone A Hanging Panel 1,               ! - Name of surface 1
   0.2,                                             ! - Flow fraction for surface 1
   ...

BuildingSurface:Detailed,
    Zone A Hanging Panel 1,        !- Name
    Ceiling,                                !- Surface Type
    Radiant Panel Construction,  !- Construction Name
    Zone A,                                !- Zone Name
    Zone,                                   !- Outside Boundary Condition
    Zone A,                                !- Outside Boundary Condition Object
    ...

Construction:InternalSource,
    Radiant Panel Construction,      !- Name
    ...

I hope this helps!