Okay, with a bit of fiddling, that did the trick. I have attempted to set PrefixMappings in my previous solutions but it appears I did it wrong then.

Output:

HSROUTER>Set tPnRXML.PrefixMappings="ns urn:oasis:names:tc:ebxml-regrep:xsd:rs:3.0"
HSROUTER>w tPnRXML.EvaluateExpression("/XMLMessage/ContentStream/ns:RegistryResponse/ns:RegistryErrorList/ns:RegistryError","@errorCode",.tPnRResult)
1
HSROUTER>w tPnRResult.Size                                                      
0
HSROUTER>w tPnRXML.EvaluateExpression("/ContentStream/ns:RegistryResponse/ns:RegistryErrorList/ns:RegistryError","@errorCode",.tPnRResult)
1
HSROUTER>w tPnRResult.Size                                                      
0
HSROUTER>w tPnRXML.EvaluateExpression("/ns:RegistryResponse/ns:RegistryErrorList/ns:RegistryError","@errorCode",.tPnRResult)
1
HSROUTER>w tPnRResult.Size                                                     
1
HSROUTER>w tPnRResult.GetAt(1).Value                                           
XDSRegistryDeprecatedDocumentError

Thank you very much.