It looks like there is no shortcut to do this, which is a shame. If you're trying to do it slightly quicker than pressing tab 8 times, you could hold ⇧ and press tab 5 times to get there in the other direction.
A somewhat more direct way is to use the third-party Shortcat utility (currently in free beta), which enables keyboard selection of graphics UI elements.
In the save dialog, you can trigger Shortcat (by default, ⇧-⌘-Space) and then start typing the name of any folder that's visible in the folder list. As soon as the folder is highlighted (and no other UI elements are), press return. You can then navigate from there with the arrow keys.
Another built-in method that's so close, but yet so far, is to press ⌘-⇧-G, then start typing the path of the folder you want. (Instead of ⌘-⇧-G, you can also press ~ to start from your home directory, or / to start from the root.) While typing the path, you can use tab (or stop typing for a moment) to autocomplete folder names.
This would work quite well if the folder list automatically updated to show the directory corresponding to the path as you entered it, but unfortunately it doesn't update until you press return. Then if you want to continue navigating, you have to press ⌘-⇧-G again to continue from where you left off.
A final possibility is to use a scripting/macro utility (like BetterTouchTool, or FastScripts) to make a global keyboard shortcut that presses tab 8 times for you. Seems a bit inelegant, though.