No - not MIVF as such. I've never actually asked for one from Freemason's but i guess the medical cert just has Freemasons logo on the top. The one i get from my FS has his details etc on the top, and the ones we have from the OPU and transfer say Royal Womens with the logo on the top.
If you are a bit worried about trying to keep things under your hat (and i understand - I sat next to a payroll person once and she couldn't keep her mouth shut over anything!) maybe you could get your cert from your doctor or your FS. They are generally very understanding and i'm sure they would help.