At 9:20 of this interview the Rammstein guitarist states they are playing Euro festivals in the summer of 2013:
It should be noted that things can change between since this interview was recorded. I recall Judas Priest reuniting with Halford and signing on to play Euro festivals in the summer of 2004 until Ozzfest had an offer they could not refuse.
I'm not sure Mayhem Fest would make Rammstein an offier like that.
Rammstein has the popularity to sell out several large venues in the Northern and West Coast of the US but I don not believe that they would fair as well in the Mid Western or Southern region of the country. A tour like Mayhem Fest could expose them to a different audience who may not have been willing to give them much of a chance.
I really think Rammstein is a possibility but not as likely as SoaD. I think SoaD, Korn or Rob Zombie will headline Mayhem Fest 2013, one of those three bands... but I'm not entirely counting Rammstein out yet.