Actually, I take it back, there is an immediate workaround, you can disable combining of CSS and it won't use the handler.
Set <add key="CombineCSS" value="false"/> in your user.config then touch Web.config so it reloads settings.
Or else, if it is possible to make /Data writable but make /Data/Sites/1/skins not writable that may also solve it.
Hope it helps,
Joe