It's currently possible (in company-managed projects) to add fields to the cards on the backlog, however this nearly doubles the height of the card, wasting a lot of space.

The only field that you can put in a card on the backlog that doesn't increase its height is the Epic Link field. This is why many people are using epics as components (the other reason is that you can filter the backlog by epic, but not by component).

It should be possible to add the component(s) on the card on the same place where epic currently shows. That way epics can be used as what they are generally in Agile literature – just big stories – and components can be used as a subdivision of projects.