On a whim I tried this on my yun:
http://en.code-bude.net/2013/02/16/how-to-increase-storage-on-tp-link-wr703n-with-extroot/
It worked! I have a 2 gig micro sd card that the yun now boots from.
The only change to the proceedure was that I had to add :
config mount
option target /home
option device /dev/sda1
option fstype ext4
option options rw,sync
option enabled 0
option enabled_fsck 0
to /etc/config/fstab