Never done it, but you should be able to change the index.html on the web service host and redirect to a particular application.
Most likely it's under c:\Program Files\QlikView\Web
To make user you can go to QMC -> Web Service -> Web -> Root Folders -> under 'QlikView'
Though, you may want to leave that functionality alone and just create html redirects.
A html redirect is a simple file that looks like this
<title>My QlikView App</title>
<body leftmargin="0px" topmargin="0px" marginwidth="0px" marginheight="0px">
<iframe seamless marginwidth="0" marginheight="0" hspace='0' align='middle' frameborder='0' height="100%" width="100%" src='http://servername/QvAJAXZfc/opendoc.htm?document=docname.qvw'></iframe>
You can create it under the Web folder, or create a separate folder and drop the redirects there. Just make sure to associate your folder under the Root Folders in Web Service properties.
Say you created a folder c:\Program Files\QlikView\Redirects and associated it under the name 'ABC', and your html redirect file is called 'MyQlikViewApp.html'.
Then you can provide your users with http://servername/ABC/MyQlikViewApp.html and presto, they are redirected to the app without ever seeing the ugly url.
This is also great when you're migrating your application between servers, just change the html redirect and done. User's don't know anything has changed.
Hope this helps.
You can append &Sheet=Document\<SheetID> to your URL. If you do a quick search you'll find that you can append a lot of things to your URL.