The backup information is stored in ~/Library/Application Support/Backup/BackupSets/. The folders in there have a unique id, so you will will have to drill down in each one to find the Backup you need. If you drill down in each one's Contents/InfoPlist.strings, you can use QuickLook to see the name in that file that corresponds to your backup set. Of course, if you make a new one, the folder with the unique id will have a creation date of today.
In the correct BackupSet folder, you will find a package User.quickpick. Show the package contents and then you will want to edit Contents/Versions/1.0/Resources/DefinitionPlist.strings. You will find a section of that file called 'Graft Paths'. From here, simply change the existing folder paths to your desired path.
Eg. pattern = ~/Documents;
change it to pattern = /Volumes/Drive/FolderToBackup;
If you have any folders within that folder to exclude, be sure to also edit the 'Prune Paths' section further in the strings file.
Eg. pattern =/Volumes/Drive/FolderToBackup/FolderToExclude;
First indications of testing it show a correct backup and restore.
[crarko adds: I haven't tested this one. It seems you need to be mindful of permissions and open files, though.]

