Access a window's frame_end row from a window function

Suppose there is the need to get the average of a value v over a 6 hours
time window starting 7 hours before the current row's time.

create table t (ts timestamp, v integer);
insert into t (ts, v) values
('2013-01-01 00:46', 2),
('2013-01-01 03:54', 4),
('2013-01-01 06:28', 4),
('2013-01-01 11:19', 2),
('2013-01-01 14:44', 1),
('2013-01-01 15:56', 5),
('2013-01-01 18:01', 4),
('2013-01-01 19:40', 0),
('2013-01-01 20:38', 5),
('2013-01-01 21:22', 0);

I can do it with a correlated subquery:

select ts, v,
(
select avg(v)
from t s
where ts between
t.ts - interval '7 hours'
and t.ts - interval '1 hour'
) average
from t
order by ts
;
ts | v | average
---------------------+---+--------------------
2013-01-01 00:46:00 | 2 |
2013-01-01 03:54:00 | 4 | 2.0000000000000000
2013-01-01 06:28:00 | 4 | 3.0000000000000000
2013-01-01 11:19:00 | 2 | 4.0000000000000000
2013-01-01 14:44:00 | 1 | 2.0000000000000000
2013-01-01 15:56:00 | 5 | 1.5000000000000000
2013-01-01 18:01:00 | 4 | 2.6666666666666667
2013-01-01 19:40:00 | 0 | 3.3333333333333333
2013-01-01 20:38:00 | 5 | 3.3333333333333333
2013-01-01 21:22:00 | 0 | 2.5000000000000000

But if I could access a window's frame_end row as a record from a window
function:

select ts,
avg(case when ts between
frame_end.ts - interval '7 hours'
and frame_end.ts - interval '1 hour'
then v else null end
) over(order by ts)
from t
order by ts

I'm naively posting this as I have no idea how complex would it be to add
this feature. Would it perform better than the correlated subquery?

Regards, Clodoaldo