I would like to create a variation of this format in which the footer is of
unknown size, but its content, while formatted top down, is aligned bottom
up. I'm sure this is trivial but I can't seem to find the magic that will do
it. The page will have content aligned top and bottom, with a variably-sized
gap in the middle. I suppose this is the kind of placement which footnotes
at the end of a chapter would have - but these are not footnotes.

You can still try to put the footer content in footnotes. The potential
tricky part is to estimate where these footnotes have to be placed in
the main flow.