I hate to say it - but I have to disagree with @dobey here - my primary reason for using Ubuntu Touch as my daily driver phone is how the fabulously intuitive native Terminal made using emacs and org-mode on UT an absolute breeze! I have an almost identical replica of emacs on UT to the one on my laptop (running Arch Linux) - and incrementally sync all of my .org files back and forth between UT and my laptop using unison (and a home server).
Install emacs on UT using apt - and then edit your .emacs file in home/phablet/ as you would usually do
As for emacs navigation, as @dobey says, you simply edit and customise one of Terminal's .json files to your liking - and then all of your classic emacs commands are on the command bar below the terminal screen. I've modified mine quite logically so that the most used emacs shortcuts appear first - in my case, CTRL+C, CTRL+X, CTRL+A etc.
To get around the missing ALT key, simply change keymappings in your .emacs file - for example, I use wanderlust in emacs for reading and writing mail and the original command to open wanderlust in emacs is "M-x wl" - so on UT, in my .emacs file, I have added:
(define-key global-map "\C-xw" 'wl)
and now "CTRL-X w" opens wanderlust
Many native emacs packages also use CTRL commands as alternatives for ALT - Dired for example (emacs built-in file browser and manager) also uses "CTRL-X d"
I have even installed texlive-full and auctex (in a chroot) on UT and used it to draft manuscripts and export to PDF - all in emacs on my phone.
So @cpb, a fully-functioning and buttery-smooth emacs experience on UT is the reason I feel like I'm carrying a fully-fledged workstation in my pocket!