I'm not sure, I'm sorry I'm just taking another look at the extension page again now, and I see that the extension is labeled as unstable. I personally usually try to avoid unstable extensions and look for a replacement that is stable rather than trying to find a workaround. I'm sorry I'm sure that's not helpful to where you currently are at this moment. I feel it is also important to mention that support ended for Mediawiki 1.31 in June of this year, normally the support staff (I'm just a MediaWiki user) here would encourage you to update to a currently supported version but I know that can be a bit of an undertaking. They might also link you to Professional development and consulting if you need help with customizing an extension, writing a hook to accomplish what you want, or upgrading your MediaWiki.
I did find Extension:PageNotice in my search, seems to do footer notices either site-wide or on individual pages I think I read, I don't know enough about your existing extension to know if it has a special page that could be linked to from that... and you wouldn't want doubling of the form on some pages for some users.
I'm sorry I can't be of more help, I found examples of hook use for LocalSettings.php to, for example, use CSS to hide items globally, I'm not super familiar with writing hooks (I cobble together simple things when needed) but targeting a single page and that way and inserting something after the content from an extension, is a bit beyond my skill level. I hope the extension maintainers or someone on the help desk with more knowledge in this area gets back to you. Strange that it would not appear on only that one page when not logged in.