default_master_connection only possible to set on session level

Hi everybody,

Imagine a multi source replication topology where "the naming convention was changed" half way implementing it. The connections have different namings.
The replication becomes inconsistent. We have to run pt-table-checksum to find the differences. With multi source replication the tool cannot find the replication topology, unless we can do --set-vars to set the SAME default master connection on all nodes. Unfortunately this is not the case in this topology.

It would be verty convenient if a DEFAULT value of default_master_connection could be set. Currently this is blocked because it's only a session variable:
ERROR 1228 (HY000): Variable 'default_master_connection' is a SESSION variable and can't be used with SET GLOBAL

Please change this so SET GLOBAL sets the default session variable (like with a lot of session variables).

Thank you,
Michael