I have not found any detailed description about it.
I made some alternative layouts: the number of buttons was changed, the layout of the keys was redesigned, but I cannot (and do not want to) add Ctrl, Alt etc. It might be interesting, but it is far from my understanding the device and the operating system.
My only purpose was to make it easier to reach the full Hungarian character set and some typography marks (and to make it easier the full ascii set in another layout). After that I made layouts for several other languages for my fun only.
Well, to tell the truth, I sometimes suffered from the French keyboard layout and I thought it would be much more comfortable in a hotel when my design would have been universally accepted:-)
You can find two scripts for installing these layouts. You must reach your device with ssh, you have to adapt its ip address, there may be further little quirks… Your measures may vary, elementary bash knowledge is necessary to edit (and then to use) the scripts…
There is a simple backup mechanism for the original files, but no guarantee is given. After trying my scripts your device may be unusable, the keyboard may disappear etc.
The scripts were successfully used on bq Aquaris E4.5 phone, M10 tablet (original Ubuntu from Canonical) and on bq Aquaris E4.5, Nexus 5 phones (recent Ubuntu Touch).
pglk is for p(h)oenician, greek, latin and cyrillic alphabets
p(oly)gl(ott) k(eyboard layout)