Yes and no.

Yes is mostly possible. No, when I was last looking at this I was told that if I roll my own image and made it available via the forums there would be 'consequences'.

I then thought, f*'it. I will keep it to myself then.

It is simple enough to generate a rootfs image from a file system image. This in turn can be turned into a flashable image by using fiasco-gen.

As of yet, NO ONE has been able to generate a flashable eMMC image with fiasco-gen. The 'layout' descriptor file format needed for fiasco-gen is unknown and lost to the mists of time. It might be possible that 0xFFFF has the ability to generate eMMC images but I never tried.

What you may consider though is there is actually NOTHING in the eMMC image of any importance. Just the n900 advert, '9' movie trailer and those crap mp3s. So with a bit of hacking it would probably be easy enough to modify the run-once-after-first-boot-script to erase the eMMC and set up the appropriate partitions etc. That way you would only need to flash the rootfs image. There is plenty of space on the rootfs image once you start stripping out all of the BS.