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!)

That would be really poor course design :-)