Actually, I just found that OpenOther works fine on iOS 6 with a little work. What doesn't seem to work is the usual way of selecting the redirect (putting the iPhone into wiggle mode, double tapping the icon, etc).
However, if you go into settings and manually insert the bundle identifiers for the apps it seems to work fine (at least on my iPhone 5 running 6.1).
For "App to Redirect": com.apple.mobilecal
For "Redirect": com.flexibits.fantastical.iphone
Hope this helps!