How's you LISP? I don't have one written, but it wouldn't take too much to write a LISP program to generate your labels - you can capture the Modelspace center (DXF code 12), the paperspace Viewport center (DXF code 10), scale (DXF code 45), height (DXF code 41) and width (DXF code 40). From those you can determine the viewport size Ht X scale, Wd X scale, and use them to get the bounding box coordinates -
XLR = CntrMSX + 1/2 WdMS
YLR = CntrMSY - 1/2 HtMS
This will only work with rectangles, but could be easily applied to all the sheets in the MapBook.
I haven't found how to get the MS center of the Viewport from the Object definition in a field to build an equation with - that would be the obvious best way - no code and completely dynamic.