I didn't know that there was an OEM drivers backrest but........if the drivers backrest bolts to the fender via the bag mounts I don't think there would be a issue.
However, there is only one back rest that I know that works with the mustang other than the one that comes with it is the one made by Grasshopper via either the site or ebay.
The passenger OEM backrest works fine as it doesn't interfere with the seats at all.
I have had all four seats that mount to the C50. Stock, OEM Gel, Mustang without backrest and now Mustang with the backrest.