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.
