By "partial" I mean it's not currently a prominent or required feature. I want to be confident nothing is breaking before it's implemented further.
Currently the only way to invoke it is to log in, go to "Your Account" and either click the blue "Update" for a character (NOT collection) or add a new character.
After that you can still manually adjust your collection if you so please (though any changes to ownership will be erased if you update your character again, as above). Once we're happy that the system is stable we'll go fully to Armory updates except for any buggy mounts which don't show up in Armory data.
Speaking of which, there's currently one known mount that doesn't show up: the Acherus Deathcharger. You'll have to ensure that's manually added to your DKs. Once it's added it won't be erased if you update again.
Please let us know if you find any other mounts that are missing from your data!

Thanks!
Wain