In the simplest case, where we'll say the exam question was precisely the topic of the 20 page paper, the candidate would be golden. Of course, it's unlikely in a 3 hr. exam that you'll be asked to write a 20 page response; but in edited form, you could definitely produce three cogent pages about some particular aspect of the original paper - if you've done the work. If you truly wrote the 20 page paper, you can surely produce three literate, cogent, responsive and topical pages.

I think schools need to set up additional, new proctoring sessions for this type of work. This will likely be something they have to hire for. A student can come and work for four hours, then hand in their in-progress draft and leave, then return later to finish it. (And please for the love of god, let students do this on offline computers, don't make them handwrite everything!)