In the Vassal mod for this game the larger Jp ground units have a 'right click' regiment creation. Now this would make it easier to garrison islands but I do not find any thing in rules of boardgame that allow it. Is this just a dream thought of by mod designer as wish list?
This is a guess as I have not yet learned how to use Vassal and I did not create the gamebox, but Mark Simonitch in c3i created some optional EoTS rules and one of them had breakdown units for garrison purposes. Perhaps the creator of the current gamebox was leaving this option open.
Just a guess...