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