Wonderful! Though I wonder why not use a simple delimiter (like MathJax does) instead of having to type out rot13(...). How about something like, for example, !!...!! or >>...<<, etc.

The chemical equations part is no longer relevant, really. mhchem is 100x better than manually saving AHK inserts - in fact, \ce typesets chemical equations almost naturally as you would type them. It's briliant! :D

I have published a more refined and improved userscript that does the same and much more: Comment keyboard shortcuts

Any chance this wonderful tool could work for non-SO sites as well?

Oh, one thing more, I just realized that the rules for the roomba user are different on the child meta sites as compared to the corresponding main site. So does this script take those into account? Or does it work on meta questions the same way it does on main site ones (which shouldn't happen)?