How can I update iTunes Match from the command line?

iTunes doesn't let you AppleScript this natively, but you can use some UI scripting to 'click' the menu bar item and do it for you:

tell application "iTunes" to activate
tell application "System Events"
    tell application process "iTunes"
        click menu item "Update iTunes Match" of menu 1 of menu bar item "Store" of menu bar 1
    end tell
end tell

You can use that via SSH:

osascript -e 'tell application "iTunes" to activate' -e 'tell application "System Events"' -e 'tell application process "iTunes"' -e 'click menu item "Update iTunes Match" of menu 1 of menu bar item "Store" of menu bar 1' -e 'end tell' -e 'end tell'