Is it possible to adjust Redactor config on a per-user basis?
Not possible with the native Rich Text field in Craft (or any field in Craft since the settings aren't defined on a per-user basis).

You could probably pull this off with a custom plugin that added a field type that extended Craft's Rich Text field and overwrote (at least) the getIntputHtml() and getSettingsHtml() methods to add per-user settings support.