Catalysis of the reaction: S-adenosyl-L-methionine + dihydroflavodoxin + -glycine = 5'-deoxyadenosine + L-methionine + flavodoxin semiquinone + -glycin-2-yl radical.
formate C-acetyltransferase-glycine dihydroflavodoxin:S-adenosyl-L-methionine oxidoreductase (S-adenosyl-L-methionine cleaving)