Once you find the folder (or any item in LB), you can assign it your custom abbreviation (press ⌘⌥A). In this case I would use "SF" for Scrivener Files.
But use whatever mnemonic works for you.
Once you have selected the folder, if you press SPACE it shows you a flat list of items in that folder. Then you can type more characters to filter that list.
The only thing missing (but maybe @Tom knows a way) for me is to filter the list by file extension, like "png" or "scriv"; OR by file type, like "image".