DONE, well, sort of.
I tweeked our rear Ultegra R603 crank alignment to allow the timing ring enough space + mud gap (more on this below) from the chainstay. However, our front Ultegra R601 crank timing alignment seems to be somewhere either left or right of the rear timing ring no matter which orientation (in or out), so I ended up with the front timing ring on the outside and then shifting the EBB a couple mm inward to achieve a close alignment with the rear ring. Having the rear timing ring mounted on the inside of the spider is probably the more important of the two, so mission accomplished there.
Initial ride with the rear ring on the inner side was last weekend when road conditions were wet and very gritty. With only 1mm or so of chainstay clearance, accumulated muck on the chainstay was rubbing the side of the belt. Having now increased the clearance to +2mm I'm hoping that will be sufficient to avoid problems in mucky conditions. Should we encounter that problem again, stoker has been instructed to squirt the chainstay with water. Also, having framesaver tape at that spot proved very worthwhile.
New observation about chainring bolts and no recesses in the CDX rings:
The Ultegra chainring bolts supplied have a backing nut depth of only 4mm, while the CDX ring + Ultegra spider totals 7mm thick. As the CDX rings do not have a bolt head recess, the rings tend to float on the spider because the ring holes are much bigger than the bolt thread width and the nut is not long enough to reach through and hold the ring in place. If these rings had head recesses, that would help hold the rings in place, but without the recess the rings move around a lot until you tighten the bolts. With the rings floating around, trying to get them perfectly centered is a real hassle.
1) Flip the bolt/nut around so the nut (now on the outside) will hold the ring in a centered position (or if you mount the ring on the inside you can leave the bolt/nut in the normal orientation)
2) use longer chainring nuts. ie: for the Ultegra spiders, 5-6mm nuts in normal orientation will reach the timing ring and hold it in place.
Your setup may vary. My comments above are based on experience with the Ultegra R601/603 cranks.