Stack trace for this problem contains:
SQL Exception: Value '0000-00-00' can not be represented as java.sql.Date
The answer? Apparently there is a hidden setting to use...
driverClassName = "com.mysql.jdbc.Driver"
url = "jdbc:mysql://localhost:3306/legacy?zeroDateTimeBehavior=convertToNull"
username = "legacyUser"
password = "legacyPassword"
That little flag on the URL is absolutely magical and now I can map my Grails objects onto a legacy database. Isn't life grand?