The question is as asked. My intended use case for this would be to have something like minipage, but whose height can be set using glue, rather than only a dimen.
When the page is pretty easy to split up, I can just use
\textheight to divide things, but sometimes there's extra text on the page, and what I really want to instruct TeX to do is just how to use the remaining space on the page, aside from the stuff I've already typed. I figure the way to do this is to put glue between boxes where content would've gone, and once the glue's been set, replace the glue with a box of the same dimensions.