I my opinion this should not be hard to make.
The big advantage you have is like you said is doing it real time.
As of i understand it, the folderdata.xml is used to make up the gallery.
With in the folderdata.xml another element could be added with a created date.
Combined with a setting within the backend "+7 days" a button could be created on that menu quite easily.
You could also make an extra function in the backend like "mark as new for 7 days" but this would be a dirty work arround which is working
i fact as I'm thinking of it right now, you could ad an extra word in the current folderdata.xml under <folder> like "updated=$date" and reflect that against the current date. so you'll get $date minus currectdate = -3 eq $button_new
But correct me if i'm wrong.
If i have some spare time I'll set it up on my own site as a test.
i'll let you know the outcome.
can you tell me which page is making the menu ? it looks like it's not flash ?