Okay, you should be able to use sfhub's Utility Program (called Auto Root Package but don't mind the name) to enable USB Mass storage on your stock ICS Touchwiz ROM.
Here's his post for reference:
xda-developers - View Single Post - [ROOT][NCIQ][EG30/EG31/EK02] Auto root your existing stock ROM (w/o ODIN/ROM Flash)
So here's what you would do. Go here and download sfhub's utility
[ROOT][NCIQ][EG30/EG31/EK02] Auto root your existing stock ROM (w/o ODIN/ROM Flash) - xda-developers. Enable USB Debugging on your phone, follow sfhub's instructions to load the utility and plug in your phone to your PC.
Then from the menu, you would select Option E, then the option to enable USB Mass Storage.
I've never done this myself, but I've used sfhub's configuration utility program many times for other things and it's excellent. It should work for you.
Let us know if you decide to try it!