Is there any reason we can't introduce a simple Webkit based help
browser that would just open a small window next to extension's dialog
on demand and show nicely formatted text? GIMP uses IDs for actions to
figure out which .html file to open in every context. Well, we do have
verbs, don't we?

And add another ~15MB dependency?

This is my only objection to the notion - it's precisely what I like to do; personally I would suggest expanding the panel to the right when the user clicks on the "help" button and including help there. That way it doesn't take more space (obscuring "live preview") unless the user is looking at the help, and the other important thing for almost all monitors, "wide, not tall", is conformed to. Visual implementation: attach another button, "Help >>" in the bottom right corner of such windows ("<< Help" when expanded).

Once you've got WebKit in there, you can also use it for other things - "preview appearance in web browser", for example, could be useful, especially for JessyInk. We could even make Inkscape play the presentation properly.

-- Chris