Are the snipers Part of the "Five Percent Nation?"
This group has a code made up of numbers which signify certain phrases- presumably passages in their pamphlets they hand out in urban areas. There is also a similar code using letters.
The group calls themselves Muslim but their ideology bears little if any comparison.
Note that Muhammad was known to pass out pamphlets when he hung out at the homeless shelter.
Note also Malvo's sketch notes, featuring apparently irrelevent numbers, as well as his text notes which feature occasional letters which are darkened by extra scribbling.
If someone has copies of said pamphlets, it may be possible to match phrases in them to some of his rambling code. Or, he may be making his own code based on the same idea.