Okay, so I know this might not be an issue often, especially since most cards on the mat are placed face-up. However, one of the workshop symbols lets you keep a card between turns. And it is possible to get a card from a face-down space on the mat.
How is this intended? Do you place all the cards face-up in front of you, or do you take them into your hand, and place the ones you have to / want to place, keeping any you are allowed to hidden in your hand?
Are you sure your question is about the trick-taking game Eternity?
I played Eternity two days ago and I don't see what you refer with "workshop" to in this game.