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An then use things like:

$ logger $'St\xe9phane'
$ journalctl --since today -o json | perl -MJSON -MData::Dumper -lne '
   BEGIN{$j = JSON->new}
   $j->incr_parse($_);
   while ($obj = $j->incr_parse) {
     $msg = $obj->{MESSAGE};
     # handle array of integer representation
     $msg = join "", map(chr, @$msg) if ref $msg eq "ARRAY";
     print $msg
   }' |
   sed -n '/phane/l'
St\351phane$

And then use things like:

$ logger $'St\xe9phane'
$ journalctl --since today -o json | perl -MJSON -lne '
   BEGIN{$j = JSON->new}
   $j->incr_parse($_);
   while ($obj = $j->incr_parse) {
     $msg = $obj->{MESSAGE};
     # handle array of integer representation
     $msg = join "", map(chr, @$msg) if ref $msg eq "ARRAY";
     print $msg
   }' |
   sed -n '/phane/l'
St\351phane$

A possible (not fully satisfactory) approach if one doesn't need to consider any of strings in the JSON as text is to pre-process the input to the JSON-processing tool (jq, mlr...) with iconv -f latin1 -t utf-8 and post-process its output with iconv -f utf-8 -t latin, that is convert all bytes >= 0x80 to the character with the corresponding Unicode code point, or in other words, consider the input as if it was encoded in latin1.

A possible (not fully satisfactory) approach if one doesn't need to consider any of the strings in the JSON as text is to pre-process the input to the JSON-processing tool (jq, mlr...) with iconv -f latin1 -t utf-8 and post-process its output with iconv -f utf-8 -t latin1, that is convert all bytes >= 0x80 to the character with the corresponding Unicode code point, or in other words, consider the input as if it was encoded in latin1.

$ exec 3> $'\x80\xff'
$ lsfd -Jp "$$" | perl -MJSON -l -0777 -ne '
   $_ = JSON->new->decode($_);
   print $_->{name} for grep {
      $_->{assoc} == 3 } @{$_->{lsfd}
   }' | sed -n l
/home/chazelas/tmp/\200\377$

Using latin1 also makes is relatively easy to deal with journalctl's representation of messages as [1, 2, 3] which we just need to convert those byte values to the character with the corresponding Unicode codepoint (and when encoded as latin1, you get the right byte back)

$ exec 3> $'\x80\xff'
$ lsfd -Jp "$$" | perl -MJSON -l -0777 -ne '
   $_ = JSON->new->decode($_);
   print $_->{name} for grep {$_->{assoc} == 3} @{$_->{lsfd}}' |
   sed -n l
/home/chazelas/tmp/\200\377$

Using latin1 also makes is relatively easy to deal with journalctl's representation of messages as [1, 2, 3] where we just need to convert those byte values to the character with the corresponding Unicode codepoint (and when encoded as latin1, you get the right byte back)