Build script classpath to an Eclipse plug-in/bundle

To avoid confusion I will use the following conventions:
I will call an Eclipse Plugin (which also is an OSGi bundle) for an Eclipse Bundle.
I will call a Gradle Plugin a Gradle Plugin.

I have custom Gradle Plugins in an Eclipse Bundle. This Eclipse Bundle is part of the Eclipse IDE installation (not a project in the workspace). The Eclipse Bundle exports the packages where the Gradle Plugins are.

How can i use my custom Gradle Plugins, located in an Eclipse Bundle, when running Gradle to build projects in the Eclipse IDE?

No matter what I try, Gradle can’t find my plugin (using apply plugin: MyGradlePlugin).

How can i include the exported packages from an Eclipse Bundle in the build script classpath?