I did a fair bit of searching but it seems that nobody skilled cares about going beyond what GMail's built-in shortcuts do. There was a Greasemonkey script, but Google integrated what it did into the current set of shortcuts and there is a Google Labs offering, but it just rebindings shortcuts that already exist to new keys.
You may, however, be able to install an extension to add keybindings to rich text areas in general. I couldn't find much, Chrome extension or userscript, for that either, unfortunately.
There is an extension called Vimium with support for custom keybindings, but since it's meant to match Vim, it's more a whole-browser keyboard navigation solution where you have to hit "i" to enter insert mode (where you can type in text areas) and then keybindings it defines won't be processed again until you hit Escape. (It lacks genuine Vim's support for keybindings in insert mode and staying in insert mode as a normal thing)
For what you want, assuming you don't develop a taste for Vimium, you'll probably have to either write your own extension (Using Chrome's ability to load Greasemonkey userscripts would give you the least boilerplate to get used to) or get a friend to do it.
If you want to give that approach a try, what you'd want to do would be:
- Find editable widgets (for jQuery, I think the selector would be either
iframe[id=':po'] but I haven't actually tried it)
- Bind a keybinding to them (If using jQuery, there are plugins to make it easy)
- When the keybinding fires, get the cursor position and modify the HTML there in whatever way you observe the Google-provided toolbar button doing it.
Sorry I couldn't be more help.