Working from this: It seems there's a limitation that when you add a link, it must be the same domain as the page.
No. This is not true. You can create a canonical link to any page that is the original regardless of what domain it is on.
So why are you seeing so many blog posts stating something else?
The answers are rather simple.
1] SEOs tend to parrot the same thing they read somewhere else. Most SEOs are not technical people and only slicing out a piece of the pie for themselves for fame or revenue. Most fail. Do not believe all that you read even from MOZ which is not on my short list of SEO sites to reference. Sorry MOZ. There are enough factual errors on MOZ.
2] Old information which may have been true or thought to be true at one point remains online forever and results in a huge misinformation campaign retained purely for traffic and revenue. MOZ has been called out for providing false information on this site before and to their credit, they have responded positively.
3] The original intent of the canonical tag was to suggest that pages on a single site were in effect duplicates or near duplicates. While that is not how the RFC was written, many SEO bloggers parroted the original motion that the canonical tag would primarily refer to the same page with parameters that offer different results. While this is good advice, bloggers rarely stepped out of the rapid blog post development mindset to add value to their posts. It is a me too(!) proposition.
It would help to read the original RFC6596 which makes it clear that a canonical link can indeed point to another domain.
- The Canonical Link Relation
..The target (canonical) IRI MAY:
...Exist on a different hostname or domain.
This is supported here:
Can this link tag be used to suggest a canonical URL on a completely
Update on 12/17/2009: The answer is yes! We now support a cross-domain rel="canonical" link element.
The original adoption of the canonical tag may not have originally considered the fact that duplicate content could exist in different domain names.
As good as Rand Fishkin is, he is completely wrong on this point. At least in today's terms. Here is what he says.
This is NOT THE CASE with the Canonical URL tag, which operates
exclusively on a single root domain (it will carry over across
subfolders and subdomains).
However, to properly defend Rand, he was probably following what Google has said and not what the RFC said. I will not dig up a link for this. Rand is good so I will make this an assumption.
Now here is the most important advice I can give you.
Be extremely careful when seeking SEO advice. Even with Rand, who is one of the best of the best, what is common, still exists on MOZ. Old posts can often be completely wrong based upon current times or even at the time the post was written. You will notice that the dates of the links I provided are all from 2009. This is important! This is far too old for current information. I would suggest reading nothing older than 2013 short of RFCs, research papers, and patents.