I face this very scenario at every gig we play
What do you do if your setlist becomes fluid? Often at the end of the night my band will start calling out songs. Hey, lets do X, Y, B, M, R. There is no setlist at that point. Song mode sounded interesting, I have never been able to see how to use it in that scenario. Am I missing something?
I face this very scenario at every gig we play. I have all my songs entered and I use SET mode for performing. I have many SETS in my MFC-101, essentially one for each of the venues or venue/genres that we perform at. My very first set though, is simply a list of all my most popular songs ordered alphabetically. My 2nd SET is my less-popular tunes, also sorted alphabetically. So if someone starts calling out random songs (Greg!)
and I don't remember it's Preset, it is very quick to change to my first SET (I called it "SORTED") and hold the switch down to scroll to the one I want. Takes about 3 seconds.
Setting this up and maintaining it is trivial with the MFC-Edit program and the main reason that I wrote it in the first place. Each SET can hold up to 50 songs and there are so many ways to use this powerful feature. You could have a SET per genre ("METAL", "ROCK", "MUZAK"

, "SORTED", whatever). It's really important to think through what you really need before setting it up, but then again, it is so easy to configure and reconfigure as your needs and preferences change if you're using one of the editors. I'll bet that there are almost as many different configurations of MFC-101's as there are owners.
If you're in the MFC-Edit program be sure to pop up the "Songs Palette" off the right-click menu - it lets you drag-n-drop files to your sets. There are lots of power-tools off the right-click menu (Insert, Delete, Move up/down, clear, copy etc.) - they're there because, for me at least, the SETS page is where the bulk of the work is. For you it may be different.
This forum is a great place to see how everyone else is doing things - I know that it was invaluable to me when I started. There's no "right way" or "wrong way" to set up your pedal - it's whatever works best for you.
-G