Persist variables added during pipeline creation

Description of the problem

When variables are specified through web interface ad hoc, it is not possible to retrieve their values after the job finished. This makes the variables interface feel incomplete: on one hand, it's nice to be able to parametrize builds ad hoc, on another hand, an important part of build's configuration is lost. Jenkins has a very similar feature (I believe, you are familiar with it, so I'm not going to describe it in detail).