a4j:status allows to define variable various content during the Ajax is in progress (with facet). So, you can define transparent of halt-transparent element over the rest content (i.e. with high z-index) that blocks the rest of the page. Usually, the ifame is used.
Similar approach is used for block the rest of the page when modal panel is shown.
Actually, you can even reuse rich:modalPanel itself if you make it completely transparent using CSS. The only blocker will work.
Try to search on this forum or google the internet. You are not the first one who ask for it.