ICloud – Is it possible to access and modify iCloud keychain from command line (or automation)

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'

Most likely, your key was imported with -x option for security import:

<...>
-x      Specify that private keys are non-extractable after being imported.
<...>

When you try to export a key, Security Framework checks whether this attribute is set and refuses to export the key.

Of course, the key is still there and you might be able to find a way to extract it (see this question).