Static DAC ports for named instances
Taken from the Microsoft Support site:


  To configure a static port for the specialized Dedicated Administrator Connection (DAC), you must update the registry key that corresponds to your instance. For example, the registry key may be the following:  
  
  HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL.X\MSSQLServer\SuperSocketNetLib\AdminConnection\Tcp
  
  Note The "X" in "MSSQL.X" is a number that indicates the directory where the instance is installed for SQL Server 2005 or the instance name for SQL Server 2008.


Although the article talks about SQL Server 2008, this works for all Windows-based versions from 2005 and above.  SQL Server on Linux uses a different mechanism, which I am not yet aware of.