any way to use indexscan to get last X values with "order by Y limit X" clause?

Similar question was
http://archives.postgresql.org/pgsql-admin/2002-05/msg00148.php, but google
did not have answer for it.

Here is the structure:

Column | Type | Modifiers
-------------+--------------------------+----------------------
id | integer | not null default '0'
datestamp | timestamp with time zone | not null
thread | integer | not null default '0'
parent | integer | not null default '0'
author | character(37) | not null default ''
subject | character(255) | not null default ''
email | character(200) | not null default ''
attachment | character(64) | default ''
host | character(50) | not null default ''
email_reply | character(1) | not null default 'N'
approved | character(1) | not null default 'N'
msgid | character(100) | not null default ''
modifystamp | integer | not null default '0'
userid | integer | not null default '0'
closed | smallint | default '0'
Indexes: tjavendanpri_key primary key btree (id),
tjavendan_approved btree (approved),
tjavendan_author btree (author),
tjavendan_datestamp btree (datestamp),
tjavendan_modifystamp btree (modifystamp),
tjavendan_msgid btree (msgid),
tjavendan_parent btree (parent),
tjavendan_subject btree (subject),
tjavendan_thread btree (thread),
tjavendan_userid btree (userid)

Here is the query:
SELECT thread, modifystamp, count(id) AS tcount, abstime(modifystamp) AS
latest, max(id) as maxid FROM tjavendan WHERE approved='Y' GROUP BY
thread, modifystamp ORDER BY modifystamp desc, thread desc limit 40

and explain analyze for it:

krtjavendan34=> EXPLAIN ANALYZE SELECT thread, modifystamp, count(id) AS
tcount, abstime(modifystamp) AS latest, max(id) as maxid FROM tjavendan
WHERE approved='Y' GROUP BY thread, modifystamp ORDER BY modifystamp desc,
thread desc limit 40;
QUERY
PLAN
----------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=18419.78..18419.88 rows=40 width=12) (actual
time=6735.06..6735.69 rows=40 loops=1)
-> Sort (cost=18419.78..18441.34 rows=8626 width=12) (actual
time=6735.04..6735.25 rows=41 loops=1)
Sort Key: modifystamp, thread
-> Aggregate (cost=16777.53..17855.84 rows=8626 width=12)
(actual time=4605.01..6711.27 rows=2938 loops=1)
-> Group (cost=16777.53..17424.52 rows=86265 width=12)
(actual time=4604.85..6164.29 rows=86265 loops=1)
-> Sort (cost=16777.53..16993.19 rows=86265
width=12) (actual time=4604.82..5130.14 rows=86265 loops=1)
Sort Key: thread, modifystamp
-> Seq Scan on tjavendan (cost=0.00..9705.31
rows=86265 width=12) (actual time=0.13..3369.28 rows=86265 loops=1)
Filter: (approved = 'Y'::bpchar)
Total runtime: 6741.12 msec
(10 rows)

This is on 7.3.3.

Having backwards reading of index would really help here.

Thanks in advance.

Tomaz