Gaurav Vaidya suggests:
This is awesome, Adam -- thanks so much for making your work freely available! Maybe we could reach a compromise in which the "Permissions" of your uploads could read, "This image was restored from [[Image:other|Image title]] and restored by [[User:Adam Cuerden|Adam Cuerden]]. The original image is in the public domain in the US/UK as its creator died in XXXX. The restored image has been licensed under a CC-BY license; in some countries, these restorations may not qualify for independent copyright and would fall into the public domain.". Then it would be up to the users as to how they choose to reuse the image. The image would be categorized and tagged as a CC-BY image, but the text would make it clear that public domain use of this image might be possible in certain jurisdictions.
The downside of this is that it'll be possible for people in the US (or anywhere else) to claim that Adam's copyright claim does not stand under UK law; however, they can do this now, and Adam doesn't have any recourse apart from the courts. However, it does record his claim, credits his work, allows users to quickly find the indisputably-public-domain image behind the restoration, and ensures that the restored images' copyright restriction of at most CC-BY is recognized. And most importantly, it makes Adam happy, so we can continue to use his modifications of these images.
I'd suggest that this seems a little excessively focused on the negative; we don't, for instance, feel the need to state that a particularly simple photograph might not be in copyright in Italy because it might be too simple and thus fall under that weird Italian law clause.
As well, at some point,a restoration is copyrightable, even under US laws. In one restoration, I combined two photographs and recreated a good 3-4% of the image. At some point, your disclaimer is going to become actively false, and I'd like some discussion on where the disclaimer gets dropped.