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The easiest way is to rename your XOOPS index.php file to something else (say xoops_index.php) and then just create a index.html page with the following:
DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Your Page Titletitle>
<meta http-equiv="REFRESH" content="0;url=http://www.the-domain-you-want-to-redirect-to.com">HEAD>
<BODY>
Please wait while we redirect you to our other site.
BODY>
HTML>
Once you're ready to "restore" your XOOPS site then all you have to do is rename the xoops_index.php file back to index.php, delete index.html and you're "back" to where you started.
NOTE: Renaming the index.php file may not be necessary depending on the server settings as the server may execute .html files before .php but the above will work in either case.
Another way to accomplish this is to clone your default template set and make it active. Then edit the cloned system_siteclosed.html file by adding
<meta http-equiv="REFRESH" content="0;url=http://www.the-domain-you-want-to-redirect-to.com">HEAD>
just before the BODY tag.