In fact you can probably find over 9000 ways to access files on a flash - scp, ftp, smb, nfs... You don't need to unmount partition to use all of the above.
No Wifi - can use usbnet (I haven't tried it with N900, worked fine with my other linux phone, http://wiki.maemo.org/USB_networking)

The only reason to use fat atm is camera and a case when you need to connect your phone to a random windows machine. 2-4Gb partition should be more than enough for that.