Hmmm... further investigation showed: this works just fine for me, as do some other tools. It worked by using a terminal emulator, which I consider way to cumbersome for file management. I thought I could just make the system partition rewritable and use any tool capable of browsing there to e.g. delete unwanted crap (like most of the ringtones). But it doesn't work that way, the tool still seems to need SU rights as well.
So I have to ask you appelflap: what is the primary purpose of this tool? If the file manager needs to be root capable on itself, I only know of Root Explorer (which can make system RW just fine, but costs money) or EStrongs File Explorer (which asks for some outrageous access rights). On a terminal emulator or ADB, I can just remount by adding a single line. Please give me a use case for your tool, is it just to save writing/pasting a single line?