Keep releasability status visible over time by showing current grades, how they've changed over time, and an aggregated grade for the product and how it's changed. The quick sketch above shows how it might look. You don't have to create it on an air-sick bag though.
About the time you get something working in each major product area, you'll be able to perform the first assessment. Create your product delivery plans so that you can do this as soon as possible, ideally before you've used half your time for delivery. Your first report card may have of lots of Ds and maybe an F or two. Don't be too concerned just yet. If you've built a thinnest working slice of your product early, you should have plenty of time to get your grades up before opening night.
At release time, your burn-down chart may not have burned all the way down. There may be a bit of functionality in there-possibly the stuff that would turn a B+ into an A or A-. But that remaining scope is just output. If you've followed the strategies above you'll have delivered on time and secured the best possible outcome for your product.
For more on story mapping see "Telling Better User Stories-Mapping the Path to Success," Better Software magazine, November/December 2009.
For more pointers on making functional quality visible, read "Simple Strategies to Keep Quality Visible."