Can you control what tags are used in an XML column? We allow our developers to use XML columns to store data.  We want to provide standard names to XML tags.

As a DBA, can we prevent the users from adding XML tags that are not predefined?
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)