I discussed this internally with the Jenkins team, and they recognize and understand what you are referring to. That is, you'd like to have specific engine credentials on the project scope and not the system settings.
At this time, the plugin only allows configuration from the system level. Since the plugin is open source, you could customize the plugin as you need. Due to the scope of the change, if you do make the change, you will probably want to branch the plugin and install it manually.
Apologies that we don't have a more immediate answer to this request, but I do hope you consider making the change in the open source project and posting back your results. It would be a great addition.
This is a link to the source code:
https://github.com/jenkinsci/delphix-plugin">https://github.com/jenkinsci/delphix-plugin">https://github.com/jenkinsci/delphix-pluginThank you,
Michael