Seems like CLOB is not supported either since the out parameter is actually bound to VARCHAR2(4000) at runtime.
The only workaround I can think of is to use an XMLType output containing only a text() node. It's still a valid XMLType but there's no tag, so it might suit your need.
create or replace procedure p1(
p_result out xmltype
passing xmlelement("test", rpad(to_clob('X'),8000,'X'))
When accessed via NDWS, the returned response is then like this :
<P_RESULT>XXXXXXXXXXXXXXXXXXXXXX ... XXXXXXXXXX</P_RESULT>
with P_RESULT length = 8000.