You'll have to excuse my L5R law being a bit rusty.
Is it common for clans to own territories far from their main land?
No, it pretty rare thoughout most of history.
In general exceptions being only temporary, e.g. occupations during a war.
The average clans has strong attachment to their ancient lands.
(though arguments (wars) about which lands are of ancient rights are theirs and certainly common enough.)
My memory is also telling me the Mantis have Enclaves, but I can find nothing about it in the wiki.
Later in history we have The Colonies and New Rokugan.
And I think all the clans took parts of that.
So given circumstances I think all are willing,
but that was a contentent sized landmass, so much value to be had.
It is my impression the Mantis, the Unicorn and of-course the Spider (who were exiled there) took major parts in the Colonies.
If it's rare, which clans would be likely to do that?
The Mantis, the Unicorn and the Spider.
Assuming it is accessible from the sea, the Mantis would be up for it.
In general the Mantis are a lot more game for adventurous action than other clans.
Mantis travel very widely by sea, so communications is less unusual for them.
Land which is not the land of other clans is expressly their responsibility.
Which has seen things like the Mantis warring in the Ivory Kingdoms.
The Unicorn
they have as a clan spent 800 years exploring the world.
And with it picked up strange gaijin ways.
They have the self-surficency to pull off seperate territory.
Odds are there actually still exist small pockets of cutoff unicorn clans folk left behind thoughout the world.
The Spider Clan.
Throughout history they have moved a lot.
From the City of the Lost, to the Ruined City, to the Fingers of Bone,
to excile in the colonies to one family breaking off and conqurying Rokugan.
The Spider, if nothing else, have demonstrated the ability to build a new home.
Even with literally every other clan out to get them.