RNA N6-methyltransferase that mediates N6-methylation of adenine of U6 small nuclear RNA (U6 snRNA). adenosine in U6 snRNA + S-adenosyl-L-methionine = H(+) + N(6)-methyladenosine in U6 snRNA + S-adenosyl-L-homocysteine Belongs to the methyltransferase superfamily. METTL16/RlmF family.