Hi I am going to be doing the same in a week or so this is interesting.  I will find a way to fix my badge on previously I have made a backing plate that can fasten on with cable ties from plastic using a dremmel if you are inventive I think maybe a plasterboard expanding fixing or something that opens up behind the grill?  I will fathom something out when my new S badge arrives.   Never guessed there was no grill access as I only just bought the XF R-Sport this week.  Cheers.