Apologies! I misread your post.
I believe that Nabble monitors the NAML macro that generates that message and you risk your forum being shut down if it is changed unless it has been altered with the knowledge and approval of Nabble.
I'm not sure where would be less obtrusive than it's current position and don't have the skills to suggest how you would turn it into a pop-up - though that seems a thing of horror to me.
However, a search of the NAML code (accessed via the "Edit this page" link) reveals that the code is in the macro "nabble_footer".
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.