Would it be best practice, for now, to use a branch or fork for projects that are getting the Warlords update treatment?
Sure, that's one method, but I was curious if I simply put the ToC at 60000, is the packager smart enough to flag an AddOn for WoD on curse.com? Or leave it as a branch and point users to it via the project description? I don't like the second method as most users either can't or don't read.
If you incorporate your changes into the trunk, and they are not backwards compatible, then you should either:
a) plan on no updates to your addon for live WoW between now and when WoD hits and just leave your updates in Alpha status (no tagging for the packager to pick up)
b) make your code backwards compatible with live
My suggestion would be to make a WoD branch. This is, after all, one of the reasons for using a version control system.
If you need to make changes that affect users on the current version of WoW, you'll be able to do so without version-checking acrobatics - just remember to merge the live branch into the WoD branch afterward, though, so you don't have an insane amount of conflicts when the time comes to merge WoD into the main branch.
The URL will always be the same for the repository - it's the same repo; a branch isn't its own repository, unless you're doing funky stuff or making a complete fork to create a new codebase. Though branches on SVN is something I've never dealt with, so I have no advice there.