Validation via XML spec file is in session level processing. So, there is not suitable hook for this.
On the other hand, there are some configuration parameters;
- UseDataDictionary : eliminates validation
- ValidateUserDefinedFields : eliminates user defined field's validation
look for detailed descriptions
edit: If your real problem is monitoring rejections, capturing Reject(3) and BusinessReject(j) messages at toAdmin() hook is sufficient.