Mount USB OTG to specific destination folder
It's actually real simple but you need work from recovery. 
All things mounted go through some script called fstab. 
And don't go change anything until you know sure that's right to do. 
Some info can be found in a map called MNT and PROC. 
However never try to alter those because it could lead to bricking your
device. 

Some textfile examples are MOUNTS and PARTITIONS where you can find detailed info. Mounting however uses the fstab folder as device parameter info point.  So think adding or changing it might work. 
Here an fstab textline as example. 

/dev/block/mmcblk0p9 /system ext4 rw


You might find more info when googling for android fstab properties. 
Still altering them in any way is up to you.