Ok, I just implemented this new control SkinFolderCssFile. Using either the latest code from our repository or the next release of mojoPortal, you can do something like this in layout.master:
<portal:SkinFolderCssFile id="css1" runat="server" CssFileName="admin.css" VisibleRoles="Admins;Content Administrators" />
where admin.css is an extra css file in the skin folder that is not included in style.config but will be linked if the user is in one of the VisibleRoles. If VisibleRoles is not specified then it is always goingn to render.
If you look at the control you'll see I also made it possible to add extra css files based on the url which should come in handy in a pinch but I would not go crazy with lots of url specific css files for different url paths.
Note that in this case the css is not processed at all by css handler but images are resolved relative the url of the css file anyway. The only reason we resolve them in css handler is because the url for csshandler does not reflect the skin path.
Hope that helps,