Nokia only...

Actually Sparsify can convert your device's rootfs image to a jffs2 for you to edit/mod but the Script is not on the original server and the dev too has forgotten about it...


Edit: Alternatively you can use dd&of to convert your phone's rootfs into a 4GB .img file...However, why do you want to edit it? Isn't it better for a stock flash image to be available for all and you can mod whatever you want above it?
Also knowing your talent I would like to request you to see if you can make improvements of the kernel itself which would be awesome... Link: http://talk.maemo.org/showthread.php?t=86543