Rather than hacking around with ItemListener (which may not work reliably when using REST, templates, etc.), better to have an instance-wide default which can be set in /configure and if necessary overridden for special jobs.

This is on my list of things to implement since a while and I was thinking to implement it the way Jesse is talking about. So that means it would go into core/maven and not into a the config-file-provider. This would then also allow to define any implementation of the SettingsProviders as default.
Also I think it would make sense to define an option where the user/admin can force to reconfigure all existing jobs (or better to remove the current configured SettingsProvider) to fall back to the global default. This would allow an admin of a bigger installation to do an easy migration.
But maybe the later would also be OK if we just provide a groovy script (maybe in scriptler).

I'm working on this here: https://github.com/imod/jenkins/tree/default-maven-settings
But I have issue to default a SettingsProvider in the Maven Builder - everything is working fine in the Maven Project, but the Builder ignores whatever I try to define as a default.
Any help appreciated...

I found the issue/solution...

pull request send: https://github.com/jenkinsci/jenkins/pull/772

Code changed in jenkins
User: imod
Path:
test/src/test/java/hudson/maven/MavenProjectTest.java
test/src/test/java/hudson/tasks/MavenTest.java
http://jenkins-ci.org/commit/jenkins/ad286c5197128e1a625c1a724350ea93b5c406a7
Log:
[FIXED JENKINS-17723] add test cases to verify defaulting global settings providers

Code changed in jenkins
User: Olivier Lamy
Path:
core/src/main/java/hudson/tasks/Maven.java
core/src/main/java/jenkins/mvn/GlobalMavenConfig.java
core/src/main/resources/hudson/tasks/Maven/config.jelly
core/src/main/resources/jenkins/mvn/GlobalMavenConfig/config.groovy
maven-plugin/src/main/java/hudson/maven/MavenModuleSet.java
test/src/test/java/hudson/maven/MavenProjectTest.java
test/src/test/java/hudson/tasks/MavenTest.java
http://jenkins-ci.org/commit/jenkins/70f457d90eed4af593fd7817f49eb6e1fa366b5b
Log:
Merge pull request #772 from imod/default-maven-settings

FIXES JENKINS-17723 - Default maven settings
Good change !

Compare: https://github.com/jenkinsci/jenkins/compare/4e37084360a3...70f457d90eed

Code changed in jenkins
User: Olivier Lamy
Path:
changelog.html
http://jenkins-ci.org/commit/jenkins/4627ec9f99d842f22c97023db00f5e3ded98d0b0
Log:
Update changelog.html

changelog entry for JENKINS-17723

Integrated in jenkins_main_trunk #2511
[FIXED JENKINS-17723] add test cases to verify defaulting global settings providers (Revision ad286c5197128e1a625c1a724350ea93b5c406a7)

Result = UNSTABLE
imod : ad286c5197128e1a625c1a724350ea93b5c406a7
Files :

• test/src/test/java/hudson/maven/MavenProjectTest.java
• test/src/test/java/hudson/tasks/MavenTest.java

Code changed in jenkins
User: Olivier Lamy
Path:
src/main/java/hudson/maven/MavenModuleSet.java
http://jenkins-ci.org/commit/maven-plugin/21fbd55cfdc857b4a33614608b29fba5b21c3bb7
Log:
Merge pull request #772 from imod/default-maven-settings

FIXES JENKINS-17723 - Default maven settings
Good change !
Originally-Committed-As: 70f457d90eed4af593fd7817f49eb6e1fa366b5b