A year or so back I built a Veronica clone using Robert Finch's 65816 core.
It never worked, but I didn't debug it at the time. I just spent a few days debugging it this work and its running well on the ÉclaireXL and the 600XL.
Details on AtariAge: http://atariage.com/forums/topic/277736-using-the-ultimate-cart-as-a-65816/
Anyway, is there any interest in a 65816 cpu in the ÉclaireXL? Now I have it running I guess it wouldn't be too hard, though I'm not sure how the linear ram access should work. Is there a standard memory map of any kind?