One thought on how to notify for low free space - Windows 10 has a notifications window, quite similar in concept to how android has notifications.
In Windows 10, notifications can appear for a short time in the lower right corner of the screen. Example: Win10 PC doing automatic backups as part of a Server essentials domain, I get a 'backup started' and 'backup completed' (or in case of an error, 'backup failed') popup. The small popup in the lower right corner of the screen goes away after a few seconds (and shows over live TV or recorded playback), but stays resident (until manually cleared) in the notifications (as witnessed by an icon in the right corner at the bottom of the screen on the taskbar. This also happens with new emails while using Microsoft Outlook, and lots of other programs. There seems to be a programable amount of time the popup lasts - the 'backup started' seems to stay on the screen for less time than a received email popup does.
In Android, most folks know how the notification section works - you get an entry and an icon in the notification section of the screen, and you have to manually clear it (or reboot the device) to make it go away.
In both there are ways to turn off or on these notifications for each application, so if you don't want to see them, you don't.
I don't know how hard it would be to program this, but since there's no real space in the UI without taking away screen real estate from other purposes, It may be an equal amount of work, or maybe even easier than changing the UI. This may work as a way to notify - particularly if there's a way to have the UI cause the popup every time you start the UI, or to not have it automatically go away unless you clear the popup manually.
Just a thought.