Gone through different threads but still have no clue.
What I want to do is to move some applications to SD card. I don't want to flash, to re-install app. Just minimum tweak because the space of the main application runs out.

My idea is to partition SD card to have 2 partitions, one VFAT (to store what's currently there), one EXT2 (for app), then move some folders in the main such as /usr/lib to EXT2 in SD card, then make a link to point there. I know I also need to modify /etc/event.d/rcS-late to reflect the partition change.

Is this doable? Any risk except I won't be able to hot-swap SD card? When partitions change, will the installed app be affected? And any other pitfalls? Thanks.

Partitioning the card that way works fine. I do it this way myself. However I'm not sure about /usr being on it and how safe that is. I use my ext2 partition for a Lenny chroot filesystem.