Perhaps you could elaborate on where you know about sqllib/tmp for the sort?
What I have found in the Data Movement Utilities pdf is:
ALLOW READ ACCESS also supports the following modifiers:
If the indexes are being rebuilt, a shadow copy of the index is
built in table space tablespace-name and copied over to the
original tablespace at the end of the load during an INDEX
COPY PHASE. Only system temporary table spaces can be
used with this option. If not specified then the shadow index
will be created in the same table space as the index object. If
the shadow copy is created in the same table space as the
index object, the copy of the shadow index object over the old
index object is instantaneous. If the shadow copy is in a
different table space from the index object a physical copy is
performed. This could involve considerable I/O and time. The
copy happens while the table is offline at the end of a load
during the INDEX COPY PHASE.
Without this option the shadow index is built in the same
table space as the original. Since both the original index and
shadow index by default reside in the same table space
simultaneously, there may be insufficient space to hold both
indexes within one table space. Using this option ensures that
you retain enough table space for the indexes.
This option is ignored if the user does not specify INDEXING
MODE REBUILD or INDEXING MODE AUTOSELECT. This
option will also be ignored if INDEXING MODE
AUTOSELECT is chosen and load chooses to incrementally
update the index.
I am also not specifying ALLOW READ ACCESS and I am issuing a LOAD ... REPLACE with indexes being auto-rebuilt and I'm gathering statistics.