I've always wondered (and googled, albeit not that in-depth) whether Mac OS is smart enough to know that your internet is being provided by a mobile network and therefore to not download software updates in the background (as mine is set to do) - I always thought that was a sure fire way to end up confused as to why all your data has gone.
OS X won't know what network it's on as far as limited vs unlimited. You can turn off auto downloading, so you just get a prompt to then download when updates are out.
Apple Menu - System Preferences - Software Update - uncheck "Download updates automatically"
Humn, that's the functionality as far as I understood it.
I just wondered that given that they know it's a tethered connection whether software update would automatically not download regardless of that particular update setting.
When I had tethering on my old carrier, I did what you suggested and disabled automatic updates, it just seemed a bit of a "backward" step given that it knows that the connection is tethered.
It's something that I think would be a good feature.