Can you control what tags are used in an XML column?
You could define a trigger in a suitable language to examine the data and block storing if validation errors occur. At least Postgres users could take this route.

For DB2 I found an example on how to use XML Schema for validation of XML columns. There should not a big difference to call XMLVALIDATE from within a check constraint or a trigger.
The example basically proposes the following steps:

-- Import the XML schema
register xmlschema 'http://sample' from 'file:///sample.xsd' as sample_db_scheme.sample_xml_schema;
-- Put schema to repository
complete xmlschema sample_db_scheme.sample_xml_schema;


Once the schema was imported, you can use it to validate the XML:

-- Place this call in a trigger or constraint
XMLVALIDATE(? ACCORDING TO XMLSCHEMA ID sample_db_scheme.sample_xml_schema)