Hey,
OSD does support ext3. ext3 is just ext2 with journalling. I'm not sure if it is Busybox being lazy and not displaying the ext3 support, or the journalling is turned off (which doesn't do harm usually).
You could try mounting it with
mount -t ext3 /dev/sda1 /mnt/tmpfs/mount_USB
yourself (making sure the /mnt/tmpfs/mount_USB directory exists & is not mounted already before you try)
-G