movem.w-loop without color#0 and using A7 is not that slow: 49 lines

MOVE.l a7,STACKSTORE

LEA CHUNKY_BUFFER,a7

MOVEM.w (a7)+,d0-d7/a0-a6
MOVEM.w d0-d7/a0-a6,PALETTES+16*2*5+2+32*0
MOVEM.w (a7)+,d0-d7/a0-a6
MOVEM.w d0-d7/a0-a6,PALETTES+16*2*5+2+32*1
... 
MOVEM.w (a7)+,d0-d7/a0-a6
MOVEM.w d0-d7/a0-a6,PALETTES+16*2*5+2+32*249

MOVE.l STACKSTORE,a7