v.nu reports no error for http://simon.html5.org/test/validator/attribute-values/img-usemap/just-a-hash.html but should, since there's no way a usemap value can be a "valid hash-name reference" unless there's at least one character in that value after the hash. Seems like the HashName.java datatype checker for common.data.hash-name needs to be made more strict. Maybe the following?

Index: HashName.java
===================================================================
--- HashName.java       (revision 404)
+++ HashName.java       (working copy)
@@ -44,6 +44,8 @@
             throw newDatatypeException("The empty string is not a valid hash-name reference.");
         } else if (literal.charAt(0) != '#') {
             throw newDatatypeException("A hash-name reference must start with \u201C#\u201D.");
+        } else if (literal.length() == 1) {
+            throw newDatatypeException("A hash-name reference must have at least one character after \u201C#\u201D.");
         }
         // Other cases
     }