
Dont know if it's too late for current beta, though.
There is no solution for KitKat [don't know about Lollipop] but for Marshmallow and above WRITE_EXTERNAL_STORAGE permission can be asked. It would bring system dialog where user can select a folder in SD (or entire SD) where app can write.
Many programs use this (file managers, ftp servers from what I encountered).
Starting point for research:
https://stackoverflow.com/questions/433 ... permission