I am migrating InstallShield projects to WiX, and I need the 'append' action available in the XmlConfig element. I'm currently using v3.6, and 'create' and 'delete' are the only supported options.
Note that in InstallShield, the removal of appended XML text on uninstall does not work properly (it removes the entire node, not just the appended text). So I use a separate search-and-replace instance to cover this during uninstall. If it would make implementation easier on the WiX side, maybe this type of thing could be done, though I don't know if WiX has a Text File Change equivalent at the moment.