Extension:PlantUML, Extension:ExternalUML, Extension:WebSequenceDiagram.
In a previous life I had these on a wiki and found that they were rarely used - the benefit was that the source was the image, so modification was easy at a later date. However people tended to prefer to generate the images in their own UML packages.
From a site point of view the more plugins you have the more maintenance you have to do.
Given that I'm under budget constraints I don't believe this is "value for money".
I do appreciate you making the suggestion though, and if you argue strongly for I can put in a backlog for the future.