Cool. It does sound like it might meet my use case. Let me know how you get on if you manage this weekend.
My weekend didn't go to plan, but I did get a little trial run.  
I managed to get to a stage where all the options:  
    [1] NORMAL
    [2] RECOVERY
    [3] ROOTED_ANDROID
    [4] DEBIAN_KDE
    [5] UBPORTS
    [6] FASTBOOT
appear on bootup, but rooted android doesn't boot.  Also note the recovery option is still there after I tried to put rooted android into 'EMPTY_RECOVERY_BOOT_2'.  
I'll have another go this weekend, maybe changing the sequence I load stuff up, or putting stuff in different slots.  
The order I tried (following the instructions from the Planet website (
https://support.planetcom.co.uk/index.php/Linux_for_Cosmo) using firmware V3) was:  
1.  Debian into EMPTY_NORMAL_BOOT_3, (worked okay)
2.  Tried Rooted Android into EMPTY_RECOVERY_BOOT_2, (appeared below recovery on boot options, but didn't boot)
3.  Rooted Android into EMPTY_NORMAL_BOOT_4, (seems okay, but android is still ugly) 

4.  Ran out of time and needed Ubuntu Touch back, so installed as per instructions here (
https://www.oesf.org/forum/index.php?topic=36413.0).  
I wrote the procedure down as I went along, it's a bit long winded for a post, but I'll pm it to you if you are interested.  
I'll have another crack this weekend if I get time.  
Sorry, can't be more help than that at the moment, I'm short of time and really don't know what I'm doing 
