Okay. Unfortunately there's no easy or obvious way to do this. But, if you can get the workflow to produce exactly the message you want (probably by using a formula tool, a Count Records tool, maybe an Append Fields tool, etc., etc.), then we can get it to work.
What you'll want to do is get to a point where you have the workflow dynamically produce the message you want. Then, feed that into a Report Text tool. In that tool, you can use the "Available Fields" drop down to have it display your message. Then, connect that to a Render tool and have it render to "...\Message.pcxml". At the completion of the app, on-screen, it will display the message to them.
Something like the attached.