Do we really want this? It's the sort of thing that has a strong potential to break anything that reads psql output --- and I'd urge you to think that human consumers of psql output may well be the minority. There's an awful lot of scripts out there.
I concede that \pset null hasn't had a huge amount of pushback, but that doesn't mean that making boolean output unpredictable will be cost-free. And the costs won't be paid by you (or me), but by people who didn't ask for it.
If we didn't use psql to produce all of our examples I'd be a bit more accepting of this position. Yes, users of it need to do so responsibly. But we have tons of pretty-presentation-oriented options in psql so, yes, I do believe this is well within its charter.
