For posterity I'll answer my own question:

I basically found a workaround using UI scripting.  It's better because the
password can stay securely stored in Keychain:

tell application "QuickTime Player"
    getURL "mms://pubint-wwno.wm.llnwd.net/pubint_wwno"
end tell

tell application "Airfoil"
    activate
end tell

try
    tell application "System Events"
        if UI elements enabled then
            tell process "Airfoil"
                set frontmost to true
            end tell
            keystroke "0" using command down
        end if
    end tell
end try