So now I have the clarification that 0xFFFF can't flash the emmc.
And flashing the emmc will erase the home partition. Furthermore until today there was no way found to integrate the home partition which also contains /opt to the fiasco image.

This means you can flash kernel + rootfs with fiasco and home + emmc needs to be dd to the device which can't be done during runtime without taking some actions. There is a way during booting the N900 to replace emmc but I would suggest using rescue os.

Dumping with 0xFFFF works and you could also use the file for flashing but you need to specify what images do you like to flash from fiasco. It is save to flash rootfs and kernel but don't flash nolo unless you know what you doing.

If you would like to participate to 0xFFFF and include flashing the emmc you could have a look here:

https://github.com/pali/0xFFFF/issues/6

As reference the explanation from Pali on irc.