Just found this, as I've been looking to do the same thing.
I just downloaded this and can confirm it works as advertised.
http://www.megaupload.com/?d=QIH5GFS8
The process is really easy and not that hard at all. Extract the files to a secure place and do the follwing.
1. Open up the folder with the files inside in an explorer window. You'll see some files and the Clean/Explicit script. Minimize this window for now.
2. Open up iTunes. Select the songs that you want to add a tag to. Make sure they're all hilighted.
3. On your taskbar bring up the explorer window containing the Clean/Explicit scripts. Now, make sure that iTunes can be seen in the background, all you gotta do now is either double click on the Explicit Script or the Clean script and you'll see some black windows pop up (it's the CMD) and they'll disappear.
4. Now all you gotta do is update the songs so iTunes can read the new tag. You can do this by simply playing the song and skip it. The new tag will be on the song and you've done it. This tag will be embed in the file and will always show up.
Removing Tags: Mistakes happen and I'm sure you've accidentally tagged a wrong song. I updated the tools with a "Remove Tag" script and it works like the other two. Select the songs and run the script.
Hope you guys enjoy this as much as I do.
Edit: If you run through some problems or need pictures, don't hesitate to send me a PM. I'm usually always on and will most likely help you out.
http://www.neoseeker.com/forums/2537/t1395307-finally-found-way-to-add-explicit-tag/