Add Feature on Join Plugin to support fully recursive projects diamonds.
For Exemple :
Root Project (0), starts Test_1, Test2 projects and join on Test_end project
Test_2 project starts Test_2_1 and Test_2_2.
Test_2_1 project starts Test_2_1_1 and Test_2_1_2.
Join project "Test_end" will be triggered only after all Test_* project will be builded.

To implement that feature i think you have to :

• Fetch project hierarchy fully recursive (on JoinTrigger)
• Build dependency graph recursively (on JoinTrigger)
• Add feature on JoinDependency to support recursive resolution (A joinDependency may have another JoinDependency).

You can find in attachment a proposal of solution implemented only for Jenkins normal relationship mechanism. It's only a proof of concept for that feature.

I hope it's usefull for you and you integrate that feature in next version of your plugin.