No command line options as far as i'm aware of, since i use the graphical ipkg installer. When asked where to install, i chose SD from the popup.
But i improved the situation by tweaking sdcontrol and fstab, so that now mmcda1 is mounted to /mnt/card2 and mmcda2 to /mnt/card. Now I'm able to install to the ext2-partition. The draw back is, that PC-Link in storage mode as well tries to get on mmcda2, witch with windows does no good.
But i can live with that. I just thought it would be nice to have things properly arranged and everything in the right place.