I spent 10 minutes searching for a simple program that would dump out the SQL that NHibernate generates from a mapping file if requested. In the end I just wrote my own. This is the relevant snippet.
NHibernate.Cfg.Configuration config = new NHibernate.Cfg.Configuration();
var export = new NHibernate.Tool.hbm2ddl.SchemaExport(config);
export.Execute(true, false, false, true);