I answered a question last week about creating a continuous but breakable rule next to an arbitrary number of paragraphs in Plain TeX. My solution gathered the paragraphs to rule in a
vbox; my output routine put that content into an
hbox and added a
vrule. If the content overflowed, I used
vsplit to place as much as a I could, then deferred the rest to the next page.
Could I have done this (in Plain TeX) without a custom output routine?