Encontrou uma resposta imediatamente após postar (o que parece acontecer com muita frequência).
In IIS, you can configure redirects on file level. From within IIS Manager, this is done by right clicking the file, click Properties and then select "A redirection to a URL". This is however impractical if you need to do this on many files, so it can of course be done from a command line or by code. Using adsutil.vbs you can use the following command lines to set a redirection on an individual file:
CSCRIPT %SYSTEMDRIVE%\Inetpub\AdminScripts\adsutil.vbs CREATE W3SVC/1/ROOT/vdir/oldFile.htm IIsWebFile
CSCRIPT %SYSTEMDRIVE%\Inetpub\AdminScripts\adsutil.vbs SET W3SVC/1/ROOT/vdir/oldFile.htm/HttpRedirect "http://www.gafvert.info"
In the example above, 1 is the website ID, vdir a virtual directory and oldFile.htm the file in the virtual directory.
Alterando minha resposta agora que testei
Na minha caixa Server 2003 / IIS 6 eu não pude usar esses comandos exatos, mas tive que executar assim para criar o arquivo virtual e então atribuir o KeyType / Properties.
SET FILE=/W3SVC/1/ROOT/test/redirection/File Path.pdf
CSCRIPT %SYSTEMDRIVE%\Inetpub\AdminScripts\adsutil.vbs CREATE "%FILE%"
CSCRIPT %SYSTEMDRIVE%\Inetpub\AdminScripts\adsutil.vbs SET "%FILE%/KeyType" "IIsWebFile"
CSCRIPT %SYSTEMDRIVE%\Inetpub\AdminScripts\adsutil.vbs SET "%FILE%/HttpRedirect" "http://www.google.com, EXACT_DESTINATION, PERMANENT"
Você também pode definir a expiração do conteúdo, os cabeçalhos personalizados etc. dessa interface.