Not sure about XMIR,
as I have not been keen on installing it
I am installing it now to test
(Ubuntu Touch phones do not come with XMIR, as yet)
but
you can certainly modify most all the system files,
The process varies from easy to insanely difficult
depending on where the files are stored.
I have used midnight commander to cruise through the system
modifying at will,
but some of the things you might want to revise
may be part of the OTA image which can really
hose your system quickly if you continue OTA updates.
What turned me off about installing XMIR on my phone
is that it looks like it would eventually require reflashing.
One good look at what's involved is notable for saying
...can run x11 apps on ubuntu touch. But there is no offical tutorial because many bugs can’t be fixed.
Anyway,
I just started as I see the Xmir package is aptgettable,
and if I can run Claws with it I will be delirious.

Xmir installs okay,
Claws installs okay,
now for the hard part
-will get back when I get some answers here