OLAC Validator custom messages can be created following these steps:
https://xerces.apache.org/xerces2-j/faq-xs.html#faq-4

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:

https://github.com/ebruchez/darius-xml.js
https://github.com/fulvio999/jxmlutil

Darius looks more promising but neither are "out of the box" tools.