OLAC Validator custom messages can be created following these steps:
This is the software it uses for its validator: https://xerces.apache.org/xerces-p/samples/validator.html Ideally this would also be containerized with the other parts.
One approach to get this containerized might be to use this script (which is older and linux oriented) https://github.com/dgricci/xmllint
Another option is to use: https://hub.docker.com/r/isaitb/xml-validator
If this service were implemented on a new server, with a web-interface we might expect to use a newer HTML front end.
here is what I found via gitub:
Darius looks more promising but neither are "out of the box" tools.