I've been trying to get this to work for the past little while and stumbled on to another openACS/Postgres bug.
This one is found in /packages/acs-kernel/acs-objects-create.sql
In the function acs_object__update_last_modified, the update statement reads:
update acs_objects
set acs_objects.last_modified = v_last_modified
where acs_objects.object_id = acs_object__update_last_modified__object_id;
This causes Postgres to give: ERROR: parser: parse error at or near "."
The correct statement should be:
update acs_objects
set last_modified = v_last_modified
where object_id = acs_object__update_last_modified__object_id;
(Let me know where else I should post this bug.)