It is possible to register paths into RequireJS (the JavaScript file and module loader used by Neos, see
http://requirejs.org) and by this custom validators into Neos. Validators should be named '<SomeType>Validator',
and can be referenced by My.Package/Public/Scripts/Validators/FooValidator
for example.
Namespaces can be registered like this in Settings.yaml:
Neos: Neos: userInterface: requireJsPathMapping: 'My.Package/Validation': 'resource://My.Package/Public/Scripts/Validators'
Registering specific validators is also possible like this:
Neos: Neos: userInterface: validators: 'My.Package/AlphanumericValidator': path: 'resource://My.Package/Public/Scripts/Validators/FooValidator'