PostgreSQL doesn't have a display type for output, so the question is just somewhat kind of silly. Using libpq you have two options
You don't get to customize how it delivers text back. And, it wouldn't make much sense either way. The library exists for people to build interfaces with, not for end users. Using C, if you're taking 1-byte for a boolean value, it's better to have
0 (the integer) and
'0' the string are fundamentally different but share the same gylph "0" and description "zero". Now for
psql I think you may have a valid feature request for that. Perhaps in the future you'll see something like
And you're welcome to open a feature request on that.
You can see this hard coded in the source here