Hi,
I think there indeed is a "LIST_TYPE_CONSTANT_SCALAR_PROC" API for UDF (Lists.F90 line 5401) as opposed to "LIST_TYPE_VARIABLE_SCALAR" that is usually used. But I think that the parameters it expects are x, y, z coordinates - not the node number. Almost all UDF's floating around are of this "VARIABLE_SCALAR" style.
I wish they would change the background of the document to make it more readable.
-Peter