A column attribute "nativeDefault" would enable a definition of a default value which is not interpreted by DdlUtils at all. The value of the attribute would be the value of the SQL DEFAULT as is.
<column name="timestamp" type="TIMESTAMP" nativeDefault="CURRENT_TIMESTAMP"/>
would result in SQL code:
ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP
With the existing column attribute default it would result in (which is not desired):
ts TIMESTAMP DEFAULT 'CURRENT_TIMESTAMP'
Comment from Tom:
In general the default value is expected to be a string value that can
be converted to the Java type corresponding to the JDBC type (as
defined by the JDBC spec).