If you have a working OE setup then you can just build a gpe-image (or opie-image? I'm not sure.)
Note that after speaking to florian (#gpe), the nokia kernel doesn't like passing control to init if it's a symlink, so you have to make sure it's presumably either a hardlink or an actual file (this is what I remember anyway.) This may have been sorted out by now (in the OE metadata), but I'm not sure.
The problems are:
* Backlight control doesn't work very well - it works (writing and reading from /sys entry) for a second or two, however then the nokia stuff grabs control back and changes the backlight up to full brightness again. To fix this, you'd need to send dbus commands to change the backlight. The format has been documented afaik, so it just requires some reasonably simple patches to minilite (or libopie2 for opie.)
* Sound doesn't work - This ought to be hackable, the dsp needs to be initialised, looking at the contents of the maemo image, this should be reasonably easy - just a case of copying scripts and the .bin file across.
* Power management - if anything keeps a clock running, the power management won't kick in (dyntick to be exact). Therefore you can't have the clock open as an applet on the gpe taskbar. I imagine other apps will also cause this kind of problem. Also note that maemo probably has far better control of the peripheral devices, so it's power management may be better (though that stuff may run from the initfs anyway so would work with a gpe-image, I'm not sure.)
* No battery power reporting - battery reporting is handled by dbus again. I think someone's decoded the readings, however it doesn't have very fine granularity which is a shame. I was wondering how easy/hard it would be to intercept the calls made to the tavho (name? iirc) device - any ideas?
Regarding getting Opie running, realistically, you'd need to add dbus support into libopie2 (which handles the hardware specifics.) I don't know how much effort this would be.
Oh yes, it may be worth writing a zaurusd style daemon to handle the lid closing events, and hardware buttons (which iirc, didn't work.)
That's about all I can remember. All in all, sounds like a bit of fun