Dynamic Margin-Bottom for Footer Content

I am using FOP 2.1. I generate intermediate format output first
and then use the concatenator to generate a PDF.

I have a footer that can be zero or more lines of content. Based on the
content, I would like the region-body to shrink in order to prevent
overlapping. This type of functionality can be seen within a <footnote>.
When present, the footnote will reserve a portion of the bottom of region-body.

I have tried to force a footnote on every page by placing it within the
before/after/start/end regions. However, I get a NullPointerException at
org.apache.fop.layoutmgr.PageBreakingAlgorithm.handleFootnotes
(PageBreakingAlgorithm.java:430).

Could I programmatically inject a footnote using the FOEventHandler API?
I haven't been able to get any code working for an override.

Is there another way that I could achieve my requirement?