Done! Running 16.04/rc now on my BQ Aquaris M10 HD.

Thanks for the help.

Steps:
Startup M10 on Android 6
Enable developer mode by tapping build number
$ adb devices
Should list the device
$ adb reboot bootloader
Select fastboot in menu
$ fastboot device
Should list the device
$ fastboot oem unlock
$ fastboot reboot

Download SP flash tool and cooler_image as described by @advocatux on https://forums.ubports.com/topic/1351/failed-to-install-ubport-on-my-bq-m10-hd/12

$ unzip SP_Flash_Tool_exe_Linux_v5.1612.00.100.zip
$ unzip cooler-image-stable-6.zip

$ cd SP_Flash_Tool_exe_Linux_v5.1612.00.100
$ sudo ./flash_tool

Scatter loading file: MT8163_Android_scatter.txt
Format with 'Auto Format Flash' and 'Format whole flash except Bootloader'
Download

Now Ubuntu Touch OTA 13 is on the device

Now ubports-installer can be run.
$ ubports-installer

After just plugging and unplugging I notice that 1 in 5 times I get an ADB connection. MTP seems to always work properly, except for one time when I got an 'Unable to open a folder for Aquaris M10 HD' error. At which time I got an ADB connection. After unmounting MTP I sometimes loose the ADB connection, but not always.

How do ADB and MTP automount play together? Do I need to disable MTP?