Since in normal mode, the text widget is actually disabled, adding keys to the keymap doesn’t work. The Vim.map() command doesn’t let you run arbitrary JS functions (though I tried pretty hard to fool it ).
The only way I could see to do something approximating what I wanted was to catch the ‘vim-keypress’ event and watch for “C-A-b”. But having done that, I could see no way to keep an actual mapping of C-A-b from firing, too. So that seems a little sub-optimal.
As an aside: Instead of “C-A-b” I’d rather map “C-S-b” or “C-B”, but the normalizer doesn’t let you map shifted letters. (As near as I can tell.)
Edit: removed < & >.