A few pieces are notated for lower notes than the usual bottom A of most pianos. I can't remember any offhand but I can think of two that arguably should be - the Bach-Busoni Chaconne, right at the end, and the Dohnanyi 3rd Rhapsody, somewhere in the middle. Both could use a bottom G. The only time I can recall hearing a bottom C on a Bösendorfer Imperial was when Ronald Stevenson tried it out at the end of Busoni's Epilogue from An die Jugend. He played it very quietly on the upbeat to the final chord, which seemed a rather lovely effect in a generous church acoustic.
Some Bösendorfers have a flap over the bottom notes, some not: some have them coloured black instead of white. The only other makes know of that extend beyond A at the bottom are Petrof and Erard - I've seen an ancient (1890-ish at a guess) Erard grand with a bottom G.