I am not aware of a cooked fw image. I don't think there will be anyone wither as it is no actual need for it. A lot of unneccessary work.

It exists scripts that would do that for you. When i have reflashed my n900 i use "apt-get" to reinstall my programs automagically. If you want to aktivate or deaktivate kernelmodules you could make a startup script.

There is a custom kernel application to tweak the mhz, volt and stuff. This you can edit while still running.. so no need to flash the device.

If you like other OS's then they could be runned out of a sd-card. This is not entierly completed though. But i believe Ubuntu and Fedora exists like this. The phone functions will not work on those what i know of though.