How about if you define the immediate field by simply multiplying the fill character by 16 megs? Assuming that the destination symbol has an implicit length, you could encapsulate this into a macro with just the destination and fill character as operands:
MACRO &L WIPE &DEST,&FILL &L LA 0,&DEST LA 1,L'&DEST LFI 2,&FILL*X'1000000' MVCL 0,2 MEND Seems to assemble okay.