As an auxiliary answer to my own question I am writing up the outcome of the chat between @scw and I on the "map-inside-addin" vs "addin-inside-mpk" approaches.
His approach would involve me modifying the makeaddin.py to also include the *.mpk (or MXD, GDB & TIF it contains) as part of the *.esriaddin.
A double-click on the add-in, extracts the folder to a place within your settings, and it'll stay there, until you either remove the add-in or update it by installing an add-in with the same name but newer 'version'.
When the add-in is installed, its extracted results are placed into
%HOMEPATH% is a Windows variable that expands to the current users' home directory). In there, should be a number of directories represented by UUID, one of which should be the add-in. Note: The AssemblyCache folder is only visible if your folder options have hide "OS-protected" files/folders unchecked (which should only be done with care).
The advantage of this approach is that there is a single step install of both data and toolbar but some Python customisation of
makeaddin.py is needed.
My approach requires me to choose to have my *.esriaddin included as an Additional File. That way as soon as I have unpacked the 2Gb map package, all I have to do is go to
%HOMEPATH%\Documents\ArcGIS\Packages\myPackage\commondata\userdata, double-click to install it and then turn on the toolbar. In other words a three step process but with no customisation needed.