New version 0.2 has been uploaded, which should solve all issues mentioned above (see also the Details tab, for more info)
Since different version of Cinema4D have different Material Manager menu I have decided to simply append a Search menu item at the end of the existing menu, instead of trying to insert it into an existing one. As this would clash with other plugins adjusting these menus as well.
I have also added a toggle button, which allows to show or hide the materials that match the entered text.
Big thanks to user bezo for beta-testing the latest version.