I just built OpenWRT for PogoPlug Pro (openwrt-oxnas-bin.tar.xz -- 73.7 MB in size). It contains just basic rootfs + Linux kernels in /boot directory. Just download and untar to extract. I have not tried it yet. I am still playing around with my PogoPlug Pro P02 and will be about to flash uboot from Bodhi. Please let me know how this goes.
[debian@debian:/opt/openwrt-git-trunk 937%] ~ ls -la bin/oxnas/ total 197004 drwxr-xr-x 3 debian user 4096 Apr 10 11:19 . drwxr-xr-x 4 debian user 4096 Apr 9 20:51 .. -rw-r--r-- 1 debian user 877 Apr 10 11:19 md5sums -rw-r--r-- 1 debian user 6993 Apr 10 02:38 openwrt-oxnas-pogoplug-pro.dtb -rw-r--r-- 1 debian user 50331648 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-ext4.img -rw-r--r-- 1 debian user 1632948 Apr 10 11:18 openwrt-oxnas-pogoplug-pro-fit-uImage.itb -rw-r--r-- 1 debian user 31588356 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-jffs2-128k.img -rw-r--r-- 1 debian user 31588356 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-jffs2-64k.img -rw-r--r-- 1 debian user 14289407 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-rootfs.tar.gz -rw-r--r-- 1 debian user 13271040 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-squashfs-sysupgrade.tar -rw-r--r-- 1 debian user 13893632 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-squashfs-ubinized.bin -rw-r--r-- 1 debian user 20613120 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-ubifs-sysupgrade.tar -rw-r--r-- 1 debian user 21233664 Apr 10 11:19 openwrt-oxnas-pogoplug-pro-ubifs-ubinized.bin -rw-r--r-- 1 debian user 1624672 Apr 10 11:18 openwrt-oxnas-uImage -rwxr-xr-x 1 debian user 1624608 Apr 10 11:18 openwrt-oxnas-zImage [debian@debian:/opt/openwrt-git-trunk 938%] ~