I'm sure there's a lot of folks who'd like to see more of the logic we have in pg_dump for building objects from the catalog available to more tools through libpgcommon- psql being one of the absolute first use-cases for exactly that (there's certainly no shortage of people who've asked how they can get a CREATE TABLE statement for a table by using psql...).
I'm all for having this as "just" as set of pg_get_*def functions, because they allow for the results to be used in queries. Granted, the shape of the result set may not be stable, but that's the sort of thing we can warn for the same way we have warnings for changes to pg_stat_activity. At that point any DESCRIBE/SHOW CREATE server side functions essentially become just shells around the pg_get_*def(), with no particular requirement to make those new commands work inside a SELECT.
Would it be totally out of left field to have the functions have an optional "version" parameter, defaulted to null, that would be used to give backwards compatible results if and when we do make a breaking change?
