Okay, I have no idea how plausible or implausible this is, because I know so little about rap music. However, I had a dream recently where someone rapped over a sample of the Soviet National Anthem about the casualties of the Great Patriotic War.
Now, obviously, that probably isn't happening, since I don't think World War 2 is a popular music topic in that category. However, how plausible is it for a rap song to sample a national anthem? I mean, okay, this has probably happened, however can a somewhat popular song plausibly do this?