discovered some more. I can set the label with the following:
object_addattr_parse(attr,"label",_jit_sym_symbol,0,""Flip/negate X values"");
Notice that I have to put quotes within the cstring. This is because the function is parsing the string and without them parses it into multiple atoms and therefore you would only see the first sequence non-whitespace characters "Flip/negate". So…quote it.