Ok, so I gave this a proper stress test last night and it worked flawlessly, barring one small thing but I doubt that's anything BackupMenu can help with - I may be mistaken though!

I backed up both rootfs and optfs using the latest Backup Menu, and created an image on the device using ubinize as described. I also exported my contacts and used the built-in backup utility to copy my communications, bookmarks etc and copied the whole MyDocs directory to PC via USB.

I then flashed the latest FIASCO image, and a modified eMMC image (with larger values for the ext3 and swap partition to make /home up to 10.5GiB and swap to 1.5GiB, leaving a MyDocs of around 18GiB). Had several issues when doing this but none related to Backup Menu so I'll leave them out (mostly me being a muppet )

After this I flashed the rootfs image I created via the -r flag, rebooted with the keyboard out to launch Backup Menu, restored the optfs tar, enabled USB host mode, copied MyDocs back and rebooted.. resulting in my system almost exactly as it was before, only with a bigger /home partition and swap. Brilliant!

The only thing missing was the power kernel, as this had been wiped when I flashed the FIASCO. However it was still installed as it had been present on rootfs/optfs, so I couldn't reinstall it. To fix this I removed it through FapMan and could then reinstall kernel-power, which then came back to life after a reboot. Is there any way Backup Menu could backup the kernel too, or is this impossible?

Thanks again for all the work Robbie, it's really an excellent utility. Oh, two more very minor points - upon reboot with the keyboard out, the ramdisk text gets mangled with whatever other text is on the left as they're on the same line, and FSCK still doesn't work. Neither of these matter a jot to me though