For anyone who might want to give this a try: I just picked up a used Dell Chromebook 13 and with GalliumOS on it, RetroPie installed using the Ubuntu instructions very easily.
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Things run very well, though GalliumOS seems to want to turn on the screen saver even if I'm using my USB controller. The workaround was to go into the power manager and enable "Presentation Mode" so the screen doesn't dim.