I've posted an update, it now has the chromium_set_string() function that will load HTML directly onto the page. You probably want to set the dummy URL as about:blank.
As transparent rendering is turned on, you may need to make sure a background color is set. There are some odd effects here I haven't quite worked out yet relating to this transparency.
Regarding CPU usage: unfortunately in order to have chromium render frames in the background and send the data over to Gamemaker via a buffer, it can't take advantage of GPU rendering, and so CPU usage is high. There's no way around this other than to have Chromium pop up as an on-screen overlay (or borderless window) placed exactly over the game, this method is used by some other Chromium GM extensions, but this method does not allow layering or shaders.