Use Tampermonkey or something to install this user script in Chrome.

extra_trello_shortcuts.js

// ==UserScript==
// @name       Extra Trello Edit Keys
// @namespace  http://www.paperlesspost.com/
// @version    0.1.1
// @description  From the card edit window, hit H to edit the description and Y to add a new comment.
// @match      https://trello.com/*
// @copyright  2012+, Alan MacDougall
// ==/UserScript==

function main() {
  $(document).ready(function() {
    console.log("Installing extra Trello keyboard shortcuts!");

    function textFieldActive() {
      var visibleFields = $("textarea:visible")
        .add("input[type='text'].field:visible")
        .add("input[type='text'].new-task-text:visible");

      if (visibleFields.length == 1 &&
          visibleFields.first().hasClass("new-comment-input") &&
          $("div.new-comment div.add-controls:visible").length === 0) {
        // only visible input field is the new comment placeholder, and comment
        // save/cancel buttons are not visible: thus no text fields are active,
        // but we ARE on a card page. Otherwise the new-comment-input field
        // would not be visible at all.
        return false;
      } else {
        return true;
      }
    }

    $(document).on("keydown", function(event) {
      if (event.which == 72) {
        // handle "h" key by editing the card description
        if (!textFieldActive()) {
          event.preventDefault(); // don't input text
          $("a.js-edit-desc").trigger("click");
        }
      } else if (event.which == 89) {
        // handle "y" key by adding a comment
        if (!textFieldActive()) {
          event.preventDefault(); // don't input text
          $("textarea.new-comment-input").trigger("focus");
        }
      }
    });
  });
}

// Inject our main script
var script = document.createElement('script');
script.type = "text/javascript";
script.textContent = '(' + main.toString() + ')();';
document.body.appendChild(script);

Version 0.1.1

Ignore the new key commands when typing a checklist item.

TODO: Ignore new key commands when date chooser, label editor, etc are open.

Micro-fix: Treat checklist item edit and checklist item add the same. Weird that they're two different classes.