Haven't tried this so not sure if this will work for you but you could try creating your own Dialect and registering that in the web.config/app.config
Dialect class:
public class MyDialect:MsSql2005Dialect
{
public MyDialect()
{
RegisterColumnType(System.Data.DbType.UInt32, "bigint");
}
}
Web.config:
configuration>
<configSections>
<section name="hibernate-configuration" type="NHibernate.Cfg.ConfigurationSectionHandler, NHibernate" />
</configSections>
<hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
<session-factory>
<property name="connection.provider">NHibernate.Connection.DriverConnectionProvider, NHibernate</property>
<property name="connection.connection_string">
Server=127.0.0.1; Initial Catalog=thedatabase; Integrated Security=SSPI
</property>
<property name="dialect">MyDialect</property>
<property name="current_session_context_class">managed_web</property>
</session-factory>
</hibernate-configuration>
<!-- other app specific config follows -->
</configuration>