Mounting SD card content read/write in Android on Chrome OS

One way to mount SD card content r/w in Android on Chrome OS is to edit the Chrome OS system file arc-sdcard.conf to pass a custom mount argument to minijail0 at boot.

As this does involve modifying a system file, rootfs verification needs to be off. Chrome OS will likely undo the modifications when it auto-updates*. Modifying system files can potentially lead to a forced powerwash and/ or USB recovery situation. It would be best to ensure any important files have been backed up. 

In any case, here's what's working for me:

Create a folder on the SD card named android-shared.

Backup the Chrome OS system file /etc/init/arc-sdcard.conf.

Add the following lines to etc/init/arc-sdcard.conf at line 69, (line 67 in some versions) - the blank space directly after the line 
args="$args -k none,/,none,0x44000" # rec,private 

If the SD card is formatted vfat (Chromebook file browser default):

if [ -d /media/*/*/android-shared ]
  then
  mkdir -p /home/chronos/user/Downloads/sd-android
  mkdir -p /home/chronos/user/Downloads/.tmpsdmount

  mount /dev/mmcblk1 /home/chronos/user/Downloads/.tmpsdmount -o umask=0000
  mount -o remount --make-rshared /home/chronos/user/Downloads/.tmpsdmount

  # Mount shared mount points.
  args="$args -b /home/chronos/user/Downloads/.tmpsdmount/android-shared,/Downloads/sd-android,1"
  # Remount to make it writable.
  args="$args -k none,/Downloads/sd-android,none,0x2e"  # remount,nosuid,nodev,noexec
fi 

e.g. SD card formatted vfat in file browser, folder "android-shared" on SD card created in file browser.
Lines added to arc-sdcard.conf at line 69, directly after the line 
args="$args -k none,/,none,0x44000"  # rec,private

If the SD card is formatted ext*:

if [ -d /media/*/*/android-shared ]
  then
  mkdir -p /home/chronos/user/Downloads/sd-android
  mkdir -p /home/chronos/user/Downloads/.tmpsdmount

  mount /dev/mmcblk1 /home/chronos/user/Downloads/.tmpsdmount -o user,rw
  mount -o remount --make-rshared /home/chronos/user/Downloads/.tmpsdmount

  # Mount shared mount points.

  args="$args -b /home/chronos/user/Downloads/.tmpsdmount/android-shared,/Downloads/sd-android,1"
  # Remount to make it writable.
  args="$args -k none,/Downloads/sd-android,none,0x2e"  # remount,nosuid,nodev,noexec
fi

e.g. SD card formatted ext2 in shell, folder "android-shared" on SD card created either in browser or in shell (if in shell chmod 777).
Lines  added to arc-sdcard.conf at line 69, directly after the line 
args="$args -k none,/,none,0x44000"  # rec,private

After a reboot, Android apps should be able to save files to /Downloads/sd-android, which should actually point to android-shared on the SD card.

Tested with various Android apps, including tTorrent and Excel.

Android apps still cannot be moved/installed to the SD card with this method. One method to gain space for Android apps is to clone the whole of Chrome OS to the SD card and run it from there with Ctrl+U in the BIOS. This seemed to run incredibly slowly on a 64GB Kingston. Somewhat faster on a 32GB Sandisk but still laggy, especially when gaming.

Note:
If any part of a command is entered incorrectly, or there otherwise is a problem with the mount -  the SD card might become unavailable and/or Android apps might fail to load  - until the card is removed, and and the device rebooted.  In addition, if you need to remove the SD card, it would be prudent to first shut down the device (or restore the original arc-sdcard.conf & reboot) before doing so.

Further Note:
For this method to work, and not crash the OS, the removable device name MUST be entered correctly, in accordance with your own personal setup.

For example, if your SD card is partitioned, you would need to substitute /dev/mmcblk1px for /dev/mmcblk1, where x is the mounted partition holding the folder to share with Android. 

Or you might have a USB drive mounted at, for instance, /dev/sda1, in which case you would need to substitute /dev/sda1 for /dev/mmcblk1. 

Especial caution is advised if using this method with a USB drive, as removing the external storage while it is still mounted in this manner may result in an OS crash.

You can check your removable device setup with the shell command:
 mount | grep media/removable

*The frequency of auto-updates, and thus, how often the procedure may need to be repeated, will depend on what release channel you are running. If you are on the Dev or Canary channel and find yourself having to regularly repeat this (or any other) modification, it might be worth considering switching to the stable or beta release channel where auto-updates are less frequent.

UPDATE: I should also note that I have a previous post on this subject, with a 'safer' shell command method (now also supports writing to ext4). If you are new to this sort of thing, it might be better to try the shell command method, as it is unlikely to crash your computer, even if you get it wrong. 

In fact, it might be worth first trying the shell command method in the previous post anyway, as the chief advantage of the method on this page was R/W access. The other difference is that this is automated at boot, for which a new upstart job could easily be created, in any case.