Well, this is a bit late, but in ChromeOS, the Colemak layout only switches the Physical search/launch button into a Backspace. You can bypass this by going to Settings>Keyboard Settings and then swapping Search onto Ctrl or Alt, but then you lose whichever you choose to use as your new search key (bad trade off). Or you could use two layouts (Qwerty and Colemak) and swap between them with ALT + Shift. So, your best bet is probably just using the launcher icon, or only switching to Colemak for longer written works.

Have not noticed a lack of repeat on the search button when used as backspace, but if you need faster backspacing, CTRL + Backspace will delete entire words at a time. Ctrl + Arrows also allows faster navigation, and works with shift for block highlighting.

Ok, there are two ways to add layouts to ChromeOS that I know of.

1) Enter developer mode and edit the xmodmap files. Many Chromebooks will reset to factory settings after exiting developer mode, so from there you can either remain in developer mode (forever), or since the hardware is unchanged you can try using the 'system recovery' option. From what I gathered, this is a bad idea and can void your warranty.

2) Create a Chrome Extension that adds layouts. I have no idea how it works, but someone did it for some  alternative greek layouts. Seems the extension adds the layout files to Chrome, then you go to the language settings to 'install' them.

(EDIT: Though, for what it is worth, Vimium works far better out of the box with Qwerty than Colemak.)

(EDIT: Also see Google- Extra Keyboards for Chrome OS on Github.)