I'm currently giving this some thought as I'm thinking of implementing something very similar on our own merchant site.
Looking at the google explanation you cited though, suggests there's no technical reason why providing a "canonical" across different domains wouldn't work.
There are situations where it's not easily possible to set up
redirects. This could be the case when you need to migrate to a new
domain name using a web server that cannot create server-side
redirects. In this case, you can use the rel="canonical" link element
to specify the exact URL of the domain preferred for indexing. While
the rel="canonical" link element is seen as a hint and not an absolute
directive, we do try to follow it where possible.
Our setup varies slightly from yours in that rather than seperate domains, we're using subdomains to internationalise the overall site. This allows us to present different converted prices to users of the different subdomains/webapps whilst still allowing us to have completely centralised management of the data.
- www.oursite.com - UK domain.
- us.oursite.com - US domain.
- eu.oursite.com - EU domain.
The problem with this setup though, is brochure/informational content such as some the helpful
guides we have on the site, which we use as content provision for the site, staging links in to products within the site. We currently have perhaps 3/4 good quality versions of exactly the same content (we haven't got the money yet to translate content, but plan to eventually).
I'm just gonna go for it in the next month or so when I've finished some core work on the site, hopefully canonical linking all of these back the "www" version of the content (just for some of the brochure info), whilst leaving products as seperate entities in their own right, in the "canonical" sense.
I'd say just go for it!
you can always switch back if it's not working as well as anticipated!