Run applescript after specific application launch

How about this: It's not exactly what you want, but achieves roughly the same thing.

I seem to remember pre-OS X you could activate an app and keep it from being frontmost, but that appears not to be possible these days (or I imagined it.)

to replaceFrontQTPlayerWithFile(aFile)

tell application "QuickTime Player"
    activate

    try
        set frontDoc to front document
        close front document
    on error err number errNum
        if errNum is -1719 then
            -- There is no open document
        else if errNum is -10000 then
            -- Front doc exists, but does not really...
        else
            log err
        end if
    end try

    open aFile
    play front document

    -- Hide QTP
    tell application "System Events"
        keystroke "h" using command down
    end tell

end tell
end replaceFrontQTPlayerWithFile


on run
    set unixFile to "/Users/diggory/Music/iTunes/iTunes Music/Music/Underworld/Oblivion With Bells/01 Crocodile.m4a"
    set macFile to POSIX file unixFile
    set fileRef to (macFile as alias)
    my replaceFrontQTPlayerWithFile(fileRef)
end run

The problem is that your Applescript runs in the context of the current user and can't access the iTunes running in other accounts. One way to close/terminate all running iTunes processes would be (in Terminal)

sudo killall -sm iTunes

but this may leave the iTunes meta data in an inconsistant state (which means longer startup time the next time you start iTunes and potential data loss).