An idea just occurred to me: a systematic way to provide this feature might be to support aliases for objects. So I could declare an alternate name for an object, something like:
CREATE ALIAS timestamp FOR TYPE timestamptz;
Which would mean that [current schema].timestamp would now be an alternate name for the built-in timestamptz object. There are other situations in which being able to define aliases would be handy, including schema migrations and probably other compatibility scenarios.
Of course I'm aware that this idea itself would need a lot of discussion and I'm not volunteering to implement it right now, but it might be a workable approach if aliases ever become a feature.
