Agreed – 777 isn’t a great set of permissions to leave around.
I’d like to suggest to the authors that they put a random number in the tmp directory name. Same as the logs file has a random number in it to make it less likely that a script hacker would find it, if the tmp directory was called (say) tmp-348712 then having it as 777 wouldn’t worry me as much.
But in answer to your question, you don’t want to be changing wp-config for this, you want to be changing the tmp directory permissions and leaving it that way.
Not ideal, but (other than writing a script that changed the directory permissions to 777 just before a backup job and then changed them back again after, or maybe doing something with group membership), I’m not sure what else we can do right now.
The Temp dir is not longer in the Plugin dir with version 3 that cames in the next days.