[Asus Zenpad 8.0 ROMs, Kernels, Recoveries, & Other] diplomatic:
This is for real, folks. The first working custom recovery for Asus ZenPad 8.0 Z380M and a method to flash it under a locked bootloader. This took quite a bit of work to make, partly due to having to find a way to bypass the locked bootloader. And because of a kernel configuration issue that limited the size of the initial ramdisk, which took some serious effort to weed out. [MENTION=4490057]lss1977[/MENTION] helped to port it to this device. And of course, credits to TeamWin for TWRP.

In addition to this thread, the current Z300M/Z301M TWRP thread and the the old Z300M recovery/rooting thread may be helpful. These models share the same hardware and software base, so most things that apply to one tablet also apply to the other.

DISCLAIMER

As usual, you do anything described in this post at your own risk. No one but you is responsible for any data loss, bricking or damage of your device.

REQUIREMENTS

Windows PC

SP Flash Tool version 5.1532 (only this version will bypass security checks)*

MediaTek VCOM drivers (available through Windows Update)

Z380M scatter file, attached

Recovery image, attached

Z380M preloader file (optional), available inside the stock firmware download

ZenPad Z380M tablet upgraded to Android N

* If you prefer to use a more recent SP Flash Tool, check out the special image file below.

This won't be a guide on how to use SP Flash Tool. There are plenty of good guides out there like this one on how to set up drivers, load scatter files, flash and make backups with SP Flash Tool. If you're not familiar with this software or how to flash MediaTek devices in general, please do your research before attempting this. I will just get straight to the specifics about the ZenPad 8.

FIRST TIME INSTALLATION

For a locked bootloader:

At this point you should have your tablet powered off, the drivers installed, all of your data backed up, and SP Flash Tool v5.1532's Download tab open and loaded with the tablet's scatter file. Open the download agent file DA_PL.bin, replacing the default MTK_AllInOne_DA.bin. (You can use the AllInOne DA if you want, but the procedure involves extra steps as described below.) Load the extracted TWRP recovery image (twrp-3.2.1-0-z380m.img) under the recovery partition line in the partition table. Click the Download button inside SPFT. Now just connect the USB cable to your tablet and your computer's USB port. It should start the download process automatically and disconnect when finished.

Alternative Method:

If you're using MTK_AllInOne_DA.bin, you have to hold the Volume Up key as you insert the cable, which puts the device into an emergency download mode. In addition, you will need to have the stock preloader*.bin file loaded inside SPFT for it to be able to communicate with your tablet. The preloader is available inside the official zip file download. Do not flash the preloader (untick the box) or any partitions other than recovery. You just need to have a valid bin file selected under the Preloader line.

For an unlocked bootloader:

If you have used the Asus unlock tool to unlock your device, you will still not be able to use fastboot flash to install a custom recovery. However, you can do a "hot boot" or a tethered boot of TWRP. This method doesn't require SP Flash Tool. Download the TWRP image, put your tablet in fastboot mode and connect to PC. Then run this command to boot TWRP dynamically:

fastboot boot twrp-3.2.1-0-z380m.img

Once inside TWRP, flash twrp-3.2.1-0-z380m.img to Recovery using Install -> Install Image. Then reboot to recovery. You may be able to use TWRP in the tethered boot mode, but that could result in glitches due to different parameters passed by the bootloader to the kernel. That's why flashing is recommended.

To update from your installed TWRP version to a new one, just transfer the image to your tablet, then boot to TWRP and install it by going to Install -> Install Image and flashing to Recovery.

START RECOVERY

With the tablet powered off, hold the Volume Up and Power keys together until you get to a menu where you can select "recovery". Use Vol. Up to scroll and Vol. Dn to select. If you have a locked bootloader, it will show a Yellow State message because the recovery is not signed by the OEM. Just press volume up to boot it. On the welcome screen of TWRP it will ask you about modifying the system partition, I suggest you go with Keep System Read-only. Doing otherwise will complicate your OTA updates.

WARNING: Never start the stock recovery from the bootloader menu on this device. It is programmed by the bootloader to instantly wipe your data without any warning. Also, be aware that Android installs the stock recovery at every normal boot cycle. So you either have to flash the TWRP every time you want to run it, or disable the automatic recovery installation in the stock firmware. Any kind of mod to the boot image will prevent the automatic recovery installation. If you're not sure which recovery you have installed, you can always do a 'adb reboot recovery' from Android without risk of data loss.

What works: Basically everything... access to major partitions, decrypted userdata partition, decrypted adoptable storage, external SD card, ADB, USB-OTG, touch interface, splash screen, installing stock Asus OTA/web FW updates

What doesn't work: (no known problems yet, but please report any)

Not guaranteed to work: factory reset

DOWNLOAD

(Updated February 3, 2018: updated kernel to firmware 5.3.18; up-to-date TWRP 3.2.1 sources)

SP Flash Tool v5.1532

Z380M Scatter file for all storage sizes (right click, Save link as...)

Recovery image

Signed image for Flash Tool only -- may be flashed with recent SP Flash Tool versions -- DO NOT INSTALL USING ANY OTHER METHOD -- tested with SPFT 5.1736.

Development and experimental files

Source code

P.S. The thanks button doesn't bite.