Persistently or aggressively enabling Battery Saver I love the Battery Saver feature. When I'm travelling or in other situations where reliable communication is more important than user experience, I want to always have it enabled, sometimes for days at a time. 

However, Android automatically turns it off after the phone has been charged, requiring me to remember to go in and re-enable it after unplugging my phone. I occasionally forget.

Is there any way (including via third-party apps, excluding via rooting or jailbreaking the device) for me to automatically or persistently enable the Battery Saver, aside from the system's built-in options to enable it at 15% or 5% charge remaining? Alternatively, is there a way to leave it on indefinitely, or a way to have it automatically activate earlier (such as at 50% charge), or a way to activate it based on a schedule?

If there's no user-accessible way to do this yet, I would also be interested in knowing if there are any mechanisms in the operating system (URL schemes, framework APIs, relevant permission settings) that could potentially be used to control Battery Saver.
automation is the preferred option. Many apps are available. I prefer using MacroDroid , since it is free (upto 5 macros) and easy to learn.
Macrodroid along with other applications like Wakelock Detector have made this possible for non rooted device using adb to grant WRITE_SECURE_SETTINGS permission to the app
Following actions are possible (quoting developer's post in the forums, accessible from within the app (10/01/17)

Battery Saver
Location Mode
NFC Enable/Disable

You need to set up adb on your laptop
Once you have a working adb connection, you need to run the following command from the command line/terminal on your computer:
adb shell pm grant com.arlosoft.macrodroid android.permission.WRITE_SECURE_SETTINGS


Your macro would look like this
Macro : Toggle Battery Saver (name of macro as example)
Trigger: Empty Trigger
Action: Battery Saver  → Toggle
Constraints: Leave blank
Once you save the macro, go to widget drawer of Macrodroid and drag the shortcut widget to your home screen, and assign to the widget the macro you have created. Press  widget to toggle Battery Saver manually
Coming to your other requirement of activating Battery Saver at user defined percentage or on full charge, create another macro
Macro :  Battery Saver on full charge or when I want (name of macro as example)
Trigger:

Battery Level → increases to 100 % ( full charge )

Battery Level → decreases to 50 % ( user defined limit )


Action: Battery Saver → on
Constraints: Leave blank
My device is rooted, so can't verify adb command but from user response it works and I had used similar command for Wake Lock Detector when my device was not rooted