David,
Try this. Touch the closed airport, and add as a Custom Waypoint. This will allow you to rename that waypoint location. Then you can add it to your flight plan and it will show as the name of the Custom Waypoint. You will also see a small banner flag with that name as well.
We actually do have it on our wishlist and I'm pushing for it. (basically to have the ability to enter your own custom waypoint by name, type, freq, comments, etc) allowing you to even add and modify currect waypoints in the database. Maybe even a "User Notes" for the official databate items. I hope this helps.