I have both the internal and external cards on my n810 formatted as ext3 and am perfectly happy. It may prevent me from using the cards as USB drives on a PC, but I've never done that anyway.

I'm not sure if there are any issues with moving a library there and symlinking it, but I can't think of any reason why there should be a problem. I do that sort of thing all the time on other systems (desktop and server).

(For grins I just moved a random library to the card on my n810, symlinked it, ran ldconfig, and loaded it via LD_PRELOAD. Seems to work just fine.)

But, what's the problem when you try to put libraries on a FAT-formatted card? The file size limit on FAT is 2GB, and I doubt any libraries exceed that. If it's a problem with permissions you should be able to overcome it with appropriate mount options. If you connect to a PC via USB though, the card will be unmounted from the n810. Depending on the library that could cause some real problems.