Is there a Hudson plugin which would allow me to create a "trend" graph for build times for all jobs/projects in a view?
It is supported for one job.
But for all jobs, the Global Build Stats Plugin will be able to do that. However, it is not there yet.
Meaning you need to develop your own plugin, based on the Plot Plugin, to vizualise that kind of trend (build times)
(source: hudson-ci.org)