Since the tool can generate SVG, you can generate a small HTML wrapper that references the generated SVG image and you're halfway there. You just need a tool window with an HTML control that you refresh when you want to. The preview tool window might be the one to tweak, you can send the information along with the generated HTML comment to the Preview tool window. That will require some tweaking. Near the end of tagwin.e / DisplayFile(), use the comments already pulled together from there using _ExtractTagComments2(), check if it has the @startuml / @enduml, send that to the planttext engine, and just add the HTML <img> link to the generated SVG to the comment text.
I'll file an FR to improve our hook code to make it easier to plug in a comment processor like this.