Balanced last page in two column output

I'm working on a stylesheet to transform a DocBook article
into the form the ACM uses for its SIG proceedings.  The
specification uses a two-column format and says, "Columns
on the last page should be made as close as possible to
equal length."

After searching around some, I couldn't find any good
references on how to do this.  Does anyone know if it
can be done, and if so, how to do it?

If it makes any difference, I'm using DocBook 5.0, the
1.73.0 stylesheets, Saxon 6.5.3, and FOP 0.93 on Linux. 

Thanks,
Dick Hamilton