Yes, that's great! Especially for
brief footnotes. Even the online Wikipedia has started to offer this lately: if you tap the footnote link on a Wikipedia page the
first time, you get a "bubble overlay" containing the text of the footnote. If you tap the same footnote link a
second time, you are then actually transferred to the footnotes section on that Wikipedia page. It would be wonderful if Marvin treated footnotes in the same way Wikipedia does it now.
Offering
only the overlay wouldn't be a good solution -- some scholarly books contain footnotes where a single footnote may be of several screens' length (!), so that displaying such a footnote in a pop-up bubble would be inconvenient. Besides, you wouldn't be able to perform standard operations with the text of the footnote in a pop-up bubble, such as highlighting and annotating it, etc.