I'm not sure how to manage this. It is possible to add exceptions for countries that don't have postal connections for an undetermined period of time. The current restrictions in Europe are temporary and things change almost daily. It would be difficult to maintain an accurate situation for each country.
From my understanding, the postal system will store those letters until delivery is resumed. So, the letters are not lost but it will take longer to be delivered. What is the case in your situation? Your postal office will store the postcards for a longer time before delivery or they don't accept your postcards at all?