Sorry, 'twiddly bits' was the best name I came up with. I'm sure there is some sort of official name for this sort of thing!
In the design, the subject is breaking boundaries: moving from its picture-like, box-like container into the 'page' itself. In the third part of the whole book project coming next month, the heraldic lion breaks the boundaries even more radically. It's a nice design conceit playing with illusion.