Thanks to jjdoctor. Works like a charm. I have a Samsung S9505, Kitkat 4.4.2, rooted with Knox 0x1.
I installed SManager from google play.
I downloaded the scripts to /storage/emulated/0/documents/scripts/massstorage
I opened SManager and navigated to the above script directory.
I selected the "Mass Storage" script.
I enabled "su" (green skull and crossbones second from left) and "bg" (looks like an &, second from right)
I selected "run"
Black screen with lines of text (it's a terminal) and it then appeared as a USB Mass Storage device.
Even does not dismount it from Android, which is nice. I hate to think how that's gonna blow up in my face at some indeterminate point in the future. Almost tempted to add some conditions so I can use it as a toggle.... but for now its awesome.
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