AFAIK, Private mode uses KNOX.
I've saw reports from S5 users who rooted their phones and lost this function.
So, i think, it nearly impossible to port because you have to root or at least flash custom ROM which will trigger KNOX flag.
Samsung Galaxy S4 SHV-E330S: Optimus ROM (TW 4.4.2) & Kernel. CPU OC 2.7GHz, GPU OC 645MHz, non-KNOX bootloader, UHD/SM/360 Camera.
Samsung Galaxy S4 GT-I9500: Optimus ROM (TW 4.4.2), Perseus Kernel. CPU No-OC, GPU OC 700MHz, non-KNOX bootloader.
Past models: Samsung Galaxy SIII SHV-E210K & GT-I9300, Samsung Galaxy SII GT-I9100.