If it wasn’t the original hard copy but a copy then I don’t think the $20 bill would be there regardless.
You are correct that getting dissertations from far off universities is often impossible. Many universities do not loan their dissertations via interlibrary loan, so unless it is online or you are willing to travel to their campus, you can't get access to it. ProQuest is another option, but they do not have everything and it costs money.
I've heard the 20 dollar bill story many times but I have never found anything like that in an original hard-copy dissertation. If I recall correctly, I have found things used as bookmarks, like receipts or scraps of paper, and the cards in the back cover are often stamped at least once a decade. That seems to suggest that other people have read them over the years, just not as often as other materials.