Plugin API: How to modify the way markdown is render

Is it possible to use the API to make a simple plugin that, for example, replaces :hw: in every block with Hello World! at render time (i.e., keeping the original file with the :hw:), without using any slash command? Just installing the plugin and every instance of :hw: visible is rendered as that other string. Of course, the rest of the text should keep standard markdown render.

I tried to use onBlockRendererSlotted, but I don’t understand the docs and I haven’t found any examples on rendering the Markdown. I then tried using provideUI inside, but it receives HTML. Can I use Markdown instead?