How to add ",a" flag to the filename delivered in maildir by procmail?

I learned that by adding ",a" to the file name in a maildir, the Evolution software considers the mail as an important message. So I would like to tag some messages using Procmail but whatever I do, I go around in circles, because I do not know the file name before it is delivered by Procmail, and when it is already delivered, it is immediately renamed by various external mechanisms by moving it from new to cur. Is there a solution only with Procmail? I know other solutions, but they do not interest me here.