The key/value-pair custom field type is currently limited to the same customisable contents as User and Version custom fields. For the key/value-pair custom field type this limitation is not necessary as the key/value-pairs are not project dependant.

The following patch removes the limitation inherited from the @RecordList@ in the @EnumerationFormat@.

--- a/lib/redmine/field_format.rb
+++ b/lib/redmine/field_format.rb
@@ -723,6 +723,8 @@ module Redmine
     end

     class EnumerationFormat < RecordList
+      self.customized_class_names = nil
+
       add 'enumeration'
       self.form_partial = 'custom_fields/formats/enumeration'

Another possibility would be to move the limitation from the parent class @RecordList@ to the child classes that need it (@UserFormat@, @VersionFormat@), but that would mean that other existing subclasses (e.g. from plugins) would need to be updated if they rely on this limitation.