The variables that are available in the toolbar.lib.ftl file are determined by the toolbar.lib.js file that lives in the same directory as toolbar.lib.ftl. Any variable that you see being set on the model object in the toolbar.lib.js file can be used in the toolbar.lib.ftl, (e.g. preferences, actionSet, googleDocsEnabled, etc.). Unfortunately the ability to create a folder is determined by the permissions set on the parent folder. In other words, if a user has write permission on the parent folder, "New Folder" button will be enabled. It's not based on the user's role in the site.
The easiest way for you to handle this without any coding would be to give the users my MyCustomRole read-only access to the parent folder. If that won't work for you, then you'll have to do some custom coding in the toolbar.lib.js file in order to retrieve the user's role for the site.