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You need to goto
Sys Admin -> Template Manger, and modify the system_siteclosed.html template.
To be able to modify template set, you will need to clone the default, and set it under
General Preferences as your template set to be used.
To display the banner, you just need to place the smarty tag
<{$xoops_banner}>
where you want the banner to be displayed.
Hope this helps...