We should have done something like that in the first place:

diff --git a/mercurial/store.py b/mercurial/store.py
--- a/mercurial/store.py
+++ b/mercurial/store.py
@@ -51,7 +51,15 @@
                 except KeyError:
                     pass
             else:
-                raise KeyError
+                if s[i] != '~':
+                    raise KeyError
+
+                try:
+                    val = int(s[i + 1: i + 3], 10)
+                except ValueError:
+                    raise KeyError
+                yield chr(val)
+                i += 3
     return (lambda s: "".join([cmap[c] for c in encodedir(s)]),
             lambda s: decodedir("".join(list(decode(s)))))
 

That would allow us to add new patterns to encode