Fwd: dbrpcparam and varchar(max) column

I'have stored procedure with output param of datatype varchar(max) and
can't find the way to pass string longer than 8000 characters.

Setting 5th parameter in dbrpcparam (maxlen) to value greater then 8000 has
no effect. In sql server profiler varchar is always declared with max 8000
in size. Setting maxlen to 0 has the same effect.
From profiler:

declare @p1 varchar(8000)
set @p1=' ... '
exec test_sp_varchar_max @p1=@p1 output
select @p1


What is the right way to use stored procedures with varchar(max) datatype.

Thanks,
Igor.

dbrpcparam reference:
http://www.freetds.org/reference/a00342.html#ga98b9b5db66975dafae8d422218e4de45