This doesn't make much sense to me, unless it's implemented 
framework-wide for any Horde_Form renderer. Adding this just to whups 
is like an arbitrary hack.
You could already create a custom controller script though, copied 
from the default ticket/index.php, that uses custom renderers for 
certain forms.

Whups can be used for many other things than tickets that need to be 
tracked. For instance I am using it as a simple docket system for 
dossiers that are being processed by various persons in my company.

Implementations like the indicated would profit much from a little 
more customization than is available via hooks and Blocks. In my case 
I do not want to bother users with certain tabs ("attachments") and 
maybe play a little with element layout and colors .

My biggest problem is to find a way to separate customization for one 
queue from a.) other queues and b.) the official file versions, so I 
can still enjoy application updates.

I suggest a simple official mechanism that allows using a dedicated 
rendering PHP file for a queue. The queue name could be passed as GET 
argument (e.g. http://<myserver>/whups/ticket/?id=123&queue=<myqueue>) 
and if there is an appropriate rendering file 
(lib/UI/renderer//myqueue.php) it is used. Otherwise, the existing 
rendering file would be employed.

I concede that this would only be quite rudimentary customization 
support; however it would allow people to quickly come up with 
renderers for special purposes without compromising system stability. 
Prettier renderers might flow back to the project.