I've just done a quick experiment with using an inline spoiler text for version history in the first post of the
Book Sync plugin. It also coincides that kpw's plugin post already "conforms" to the requirements. Quite simply they are that you have a line containing "Version History" (case insensitive) on the line preceding the Spoiler with your text. Inside the spoiler put whatever text you like.
See the attached screenshots for the changes I made to the GUI - the contents of your spoiler tag are rendered in the same html in the popup window. The Version History button is enabled based on the presence of a value of "History: Yes" (case-insensitive) in the plugins sticky index.
Any thoughts? This may all be an interim solution until we move the plugins to a repository but it is simple enough for the meantime. I think the spoiler is more convenient for forum users than additional popup windows from attachments and it will be faster to display from the plugin as one less retrieval.