There was a monthly compo suggestion along these lines. That was inspired by the creature in Gears of War II which a section of the game is played within. I was also reading an Ian M Banks book (again...) "Look to Windward" which had these colossal creatures called Megafauna with cavities and pockets inside them where whole communities live symbiotically. So where I thought that the idea had only a special interest I believe it might have more general usage now.

My only downplay on the idea is that I did the Werthers skull map a while back - a hideout set inside of a titan or god like skull - and wanted chambers all rounded with ligaments and so on. Very hard to map...