Standard metadata queries

I've been chatting with the Trolltech folks about the implementation
of the Qt wrapper around libpq, and the issue of how to properly do
metadata queries came up. That is things like "What are the column
names and types of the primary key of this table, and what index
enforces it?" or "What are the names and types of each field of this
table?".

These seem like queries that'll be used by a lot of people, hidden
down in ORMs and access libraries, and which are hard to get right,
let alone efficient, and which will generally be written by one person
(developing the ORM or library) and likely not touched again.

Is there a standard set of well-crafted implementations of these
anywhere that could be used by all the interface and ORM developers?
If not, would it make sense to put some together and document or wiki
them? Both as example code and as a set of good, solid queries that
library developers can cut and paste.

(The implementation I'm looking at right now has, amongst other
things, hardwired OID-to-type mappings, and there's got to be a
cleaner way than that).

Cheers,
Steve