Yes this is doable, with the caveat that I have not invested a lot of time in documenting all the plug APIs etc. You can have a look at the mermaid plug to get a sense of how this can be done, it will be similar except that you — indeed — may end up having to post something to a URL somewhere rather than render the thing on-the-fly with a JavaScript library you load externally: github.com/silverbulletmd/silverbullet-mermaid