It sounds like a temporary fix for the keychain hack on iOS would be to just never use the SecItemUpdate keychain API, and always use SecItemDelete followed by SecItemAdd with the updated data which according to http://opensource.apple.com/source/Security/Security-55471/s...:
> @constant kSecAttrAccessGroup
...Unless a specific access group is provided as the value of kSecAttrAccessGroup when SecItemAdd is called, new items are created in the application's default access group.
If I understand this correctly that would always make sure that when an existing entry is updated in an app, the 'hack' app would again be restricted in being able to access the entry's data. It could still clear the data, but wouldn't be able to access the contents.
The paper seems to note this as well:
> It turns out that all of [the apps] can be easily
attacked except todo Cloud and Contacts Sync For Google
Gmail, which delete their current keychain items and create
new ones before updating their data. Note that this practice
(deleting an existing item) is actually discouraged by Apple,
which suggests to modify the item instead [9].