Old Ways Are Best!
I'm sure I'm just missing it, but I am seeing nothing but common sense preventing me from using Galleys in land combat.
Looking at the card matchup spreadsheet was my first clue that Galleys can't do land combat, as the cards that can't do sea combat are greyed out in all the Galley cross-references.
That's correct and an application of common sense as much as anything. Page three describes land battles and, as you can see, there is no reference to the Galley cards as they are not used in land combat. Over the page where Sea Movement and Battles are described, you will find the paragraph where the use of Galley cards comes into play along with restrictions on which and how many of the other cards you can use.