Deferral of primary key constraint

Is it possible to defer the check on a primary key constraint (or a
check constraint, for that matter). Here is an example that shows why
it would be nice to be able to do so. We have a real-life scenario that
is similar (but much more complex).

drop table btab;
create table btab (
base varchar,
pos int4,
cmt varchar,

primary key (base, pos)
);

insert into btab (base,pos,cmt) values ('aa',1,'The');
insert into btab (base,pos,cmt) values ('aa',2,'quick');
insert into btab (base,pos,cmt) values ('aa',3,'grey');
insert into btab (base,pos,cmt) values ('aa',4,'fox');

insert into btab (base,pos,cmt) values ('bb',3,'dog');
insert into btab (base,pos,cmt) values ('bb',2,'brown');
insert into btab (base,pos,cmt) values ('bb',1,'The');

select * from btab order by base,pos;

begin;
delete from btab where base = 'aa' and pos = 2;
update btab set pos = pos - 1 where pos > 2 and base = 'aa';
commit;

select * from btab order by base,pos;

begin;
update btab set pos = pos + 1 where pos >= 2 and base = 'bb';
insert into btab (base,pos,cmt) values ('bb',2,'slow');
commit;

select * from btab order by base,pos;

The last transaction fails (on my box, anyway) because of the primary
key index.

We would like to be able to do inserts/deletes to a list of records and
still ensure that they are in contiguous sequential order so we want to
renumber higher records if a new record is inserted in the middle. The
sequence is part of the primary key and we want to ensure uniqueness.
Some renumbering will work (by chance) if the records happen to get
adjusted in the right order. But if one of the updates tries to rename
to an already existing record, it fails.

How hard is it to take the deferral mechanism you have for foreign key
references and apply it to the primary key too? It would also be handy
to be able to defer a check constraint.