GNOME bug 102548 suggests that warning dialogues should have a special frame style, and it’s suggested that this could look like safety tape wrapped around the edge.
This is not unlike the special frame style suggested here for root windows. However, while there’s already a way for the window manager to tell whether a window belongs to the superuser, there’s currently no way to tell whether a window is a warning, so this would need a change made to the EWMH and then need all the toolkits fixing to use it. It’s thus rather nontrivial, although it may still be worth it if it helps users.
I am seriously entertaining the idea of doing away with the window and frame_style_set tags in v3 of the format, and just using tags on frame styles such as maximized, shaded, focused, unfocused, root, warning, modal, and so on, with some well-defined and intuitive rule about how to break ties:
<frame_style geometry="foo" tags="border focused maximized">
<piece position="title"> ...
Photo © Robin Gallagher, cc-by.