I think this has potential, but it is too liberal. There are many more
characters that cannot be assumed printable, e.g. many of the Latin-1
characters in the range 0x80 through 0x9F.  Isn't there some Unicode
data table that shows code points that are safely printable?

OTOH there are other potential use cases where it would be nice to see
the \u escapes, e.g. when one is concerned about sequences that print
the same but don't have the same content (e.g. pre-normalization).

The backslashreplace trick is nice, I didn't even know about that. :-)