S-adenosyl-L-methionine-dependent methyltransferase that mediates mRNA cap 2'-O-ribose methylation to the 5'-cap structure of late viral transcripts. a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyl-ribonucleoside) in mRNA + H(+) + S-adenosyl-L-homocysteine