to_char_at_timezone()?

Hi,

9.4 FINALLY added the UTC offset formatting pattern to to_char().
However, it falls a bit short in the sense that it's always the time
zone offset according to the effective TimeZone value. This has a few
issues as far as I can tell:

1) It's not truly controlled by the query which produces the
timestamptz values in the case of e.g. functions
2) Having to SET LOCAL before a query is quite ugly
3) It supports only a single TimeZone value per query

So I got into thinking whether it would make sense to provide a new
function, say, to_char_at_timezone() to solve this problem. For example:

local:marko=#* select now();
now
-------------------------------
2014-11-05 00:43:47.954662+01
(1 row)

local:marko=#* select to_char_at_timezone(now(), 'YYYY-MM-DD
HH24:MI:SSOF', 'Etc/Utc');
to_char_at_timezone
------------------------
2014-11-04 23:43:47+00
(1 row)

local:marko=#* select to_char_at_timezone(now(), 'YYYY-MM-DD
HH24:MI:SSOF', 'America/Los_Angeles');
to_char_at_timezone
------------------------
2014-11-04 15:43:47-08
(1 row)

Any thoughts? The patch is quite trivial.

.marko