Launch Apple Script on App Focus

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

duti is a shell utility that enables using a text file to configure the default applications for file types and URL schemes.

For example save a file like this as ~/.duti:

com.gnu.Emacs public.plain-text all 
com.gnu.Emacs public.unix-executable all
org.videolan.vlc .mkv all

Then run duti ~/.duti.

You can install duti with brew install duti or by running wget https://github.com/fitterhappier/duti/archive/duti-1.5.2.tar.gz;tar -xf duti-1.5.2.tar.gz;cd duti-duti-1.5.2;./configure;make;sudo make install.