i just saw your post now, i could not get movep to work in Seka.
but what you describe is not what movep does.
movep work like if you put a value in a address register d0 example #$ffff,d0
and you have $78000, in a0
and you use movep d0,(A0)
you will get $ff in 78000 and $ff in 78002, if you do the same in 78001 you will put data in 78001 and 78003, if you movep.l
if will put data in 78000, 78002, 78004, 78006 and the same if you do in on a odd address it will move datas to 78001,78003, 78005, 78007.
and that is total super on how the video memory is on the Atari ST.
if you store you graphic right you can move 8 pixel to all bit planes in one go