Why wouldn't they accept the coins? I know they tried to block "tainted" coins for a while in 2011. Those were coins that had been reported stolen, but they gave that up pretty quick, because it's basically impossible. The issue here isn't whether it's possible to transfer coins from SR to MtGox, it certainly is. The issue is whether that is safe. Someone investigating transactions in the block chain, who has identified bitcoin addresses controlled by SR, could figure out that you're making money on the site, and MtGox would have no problem turning over your info in response to a proper LE request.