Yes, the 28 days number have "lots" of safety margin build in. I do not think you will ever get any company or doctor on record that will tell you how to use it over that magical number.
With that said, I personally did not run into that situation before. So I am no help. When I was on Levemir (basal injection) for a short period, I will use up one pen less than 10 days. Now I am on Novolog in my pump, I use one bottle less than 2 weeks.
Others will chime in soon