KissMetrics' idea of "json" isn't very canon. This hack mends json-illegal octal escapes.

jsonish-to-json.pike

#! /usr/bin/env pike

inherit Tools.Standalone.process_files;

string version = "1.0";
string description = "Replaces \\oct escapes with \\u00XX escapes.";
string usage = #"[options] <files>

jsonish-to-json is specifically suited to mending KissMetrics \"json\" dumps.
Available options:
" + default_flag_docs;

string encode(int c) {
  switch (c) {
    case '\b': case '\t': case '\n': case '\f': case '\r':
    case 32..127: return (string)({ c });
    default: return sprintf("\\u00%2x", c);
  }
}

string process(string jsonish) {
  mapping changes = ([]);
  for (int i = 255; i >= 0; i--) {
    string c = (string)({ i }), e = encode(i);
    if (c == e || c == "\\") continue; // no need to change encoding of these
    changes[c] = e;
    changes[sprintf("\\00%o", i)] = e;
    changes[sprintf("\\0%o", i)] = e;
    changes[sprintf("\\%o", i)] = e;
  }
  return replace(jsonish, changes);
}