Apparently, service-builder's DTD is incomplete, even though service-builder is explicitly aware of some settings. From this forum post:
Unable to create entity because column name exceeds 30 characters. Some databases do not allow column names longer than 30 characters. To disable this warning set the "service-builder" attribute "database-name-max-length" to the max length that your database supports.
The attribute database-name-max-length however is not documented in the DTD.