Hmm.... what about copying the whole /usr/bin directory(or at least parts of it) to a place somewhere on th e mounted sd/mmc card and then place a symlink , like: /usr/bin -> /mnt/card1/apps/bin. If this is going to make problems, then one can always just copy big files like libraries and databases to the card and then link them....

edit: Or one can use the --bind parameter in fstab.