Hi EArkham, I answer here too:
When you search at "root" level of the library, you'll get all compatible results.
To only display results in "Filters", you'll need to select the group first.
In the case you have all your materials at the root level of your custom folder, you can uncheck the "Recursive" option in the "Paths added" list of your Project preferences. This way, the subfolders will not be displayed.
If you don't need the bitmaps of your material folder, you can use the library filter, and specify to only display "Graph" (instead of all). This way, you'll only get your Substances.