Can default object names be changed? Sooner or later, a db developer will come across, and possibly use, a naming scheme for many objects like foreign keyes, default constraints etc.

Let's consider this example: Name all foreign keys as:
FK__[name of source table]__[name of source col]__ref__[name of master table]__[name of master column]

What kind of parametrization/external programs could be used to achieve having such a name as default, when for example using a create table statements which mentions the fk but does not explicitly provide a constraint name?
What kind of parametrization/external programs could be used to
  achieve having such a name as default, when for example using a create
  table statements which mentions the fk but does not explicitly provide
  a constraint name?


There's no way to change the default name generated when the DDL doesn't contain a name.  The only thing you can do is to run a batch process to identify non-compliant objects and perhaps drop and recreate them.