I will add some tests i have made:
I created a custom application with / as context root that replace the ROOT.war. This app has a custom 404 page. The other webapps deployed on that server all have a custom 404 page.
This works most of the time as with visiting a wrong context or wrong page, BUT it doesn't when requiring a wrong resource, for example a wrong png. In this last example the default 404 page is displayed.