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