In answer to your question yes .9. pngs can be modified but they require a special method.
They are first modified to your likings in whatever paint application you want to use. Example, Gimp, Paint Shop Pro, etc.
Then they have to be run through "DrawPatch" which is a tool and part of the Android SDK package which is available to anyone. This will allow you to format the file properly to a 9 png file that will compile properly.
You have to then slip this file, in your case "statusbar_background.9.png" into the "framework-res.apk" of your ROM or a flashable zip and flash it in CWM.
Sounds not to bad but the problem is, whatever framework-res.apk/twframework-res.apk package you use say from the Emotionless Beast theme could overwrite you have already changed previously. Simply flashing a change to the statusbar itself is simple if you know what to expect from the package your using but otherwise it could once flashed change something else that is themed in your device with a "newer" file/image/png, etc.. if you get what I'm saying.
So I hope this helps you along in your quest for changing your status bar. Its not a hard deal really but if you were to use a newer modified status bar image in the Emtionless Beast theme you have it will reset whatever changes you have made to the E B theme since the first time you flashed it.