Lots of (most?) dead keys on OS X are currently non-functional. The bugs depend on the layout chosen, e.g macron works with ABC Extended, but not US International. Some also send the wrong keysym (diaresis instead of double acute). The first layouts that should be sorted out are ABC Extended and US International as they seem to exhibit most of the issues.
GTK has some handling of this as well, so we can probably look at what mappings they've identified. See gdk/quartz/gdkkeys-quartz.c.
