For sure this cannot be public. The method isn't needed at all, as the field is visible to the only caller.

I don't think even the static field is needed. The only use I see is createManagementStatisticsResource and that is only called from a DUP. The DUP has access to the DeploymentUnit and can use DeploymentModelUtils.MUTABLE_REGISTRATION_ATTACHMENT to get access to the root ManagementResourceRegistration for the deployment.