I just did this same process in the last few days (including needing to find old meego rescue initrd in the archives). I made dd images of all my eMMC partitions and the rootfs ubi image as well.

To get the rootfs image boot into meego or nitdroid and copy /dev/mtd5 to a file on your SD card.

You can flash it back to your N900 later, or on your linux computer you can mount it and get your files out:
http://talk.maemo.org/showpost.php?p=403015&postcount=3

with rootfs + emmc you should be safe to completely reflash your device for mailing, and then restore it back to the same state later (if it's a hardware problem).