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I am not quite sure why you need to use a subfolder-- if you go into XOOPS preferences and 'turn your site off,' then logout or go to the site with a different browser, you get effectively what you are looking for. A plain page (depending on the template, you can edit it) with a log in screen and a message (which you can also change).
There is no real reason to need a "Xoops" subfolder, as no one can get in without a login anyway.
Try that out and see if it doesn't work for you.
T