How to mask USB OTG as SD card
I found a rather easy solution here, but it requires Android Marshmallow 6.0:

(My Device is a rooted Nexus 7 2012 with latest ASOP 7.1.2 installed, the OTG Device is a USB-Card-Reader with a Micro-SD-Card).
I'm note sure if you need root for these commands.

Connect your OTG-Device, then run the command in a Terminal-Emulator (like this one) directly on your device:

sm set-force-adoptable true


or use ADB from your pc (you'll need to install the Android Debug Bridge ADB first, see here):

adb shell sm set-force-adoptable true


After that, you should see a Notification, which ask if you want to use the storage "normally" or as adoptable storage (which would format the Usb-device). I chose the first one (without formatting), then the my OTG-Device was available like a Micro-SD-Card on my SGS5.
The OTG-Device will then automatically be mounted under /storage/0000-ffff (hex-code of your device).

Now i can choose my "new" SD-Card for Netflix Downloads. I assume Youtube offline downloads are supported now, too.

There is a Notification to "discard" your device, with that I even can disconnect the OTG-Device and/or restart my android, the next time I reconnected my OTG-Device I was able to use the Netflix Downloads again, without repeating the commands (sometimes I had to force-close Netflix first)!

With my OTG-Adaptor (Inateck OTG Hub &amp; SD Card Reader HB3001G) I even can use OTG and charge at the same time (connect the power source to the OTG Reader first, then to your Android Device, for some reason that dosen't work all thetime)!