I found 2 Chinese threads talking about running Kobo native ereader, koreader, and android on Kobo Aura HD. People say it is easy to switch from one to another smoothly. Unfortunately, I am technologically challenged so I couldn't figure out how it was done - there is no details (not enough for me) in the discussion. I am posting these two links here for people who can read Chinese and know android system.
Here and
here.
And
this blog article about Debian/Android DualBoot for Kobo Aura HD, also in Chinese. There is some "boot setting" in a screen shot, kind of similar to the idea of a menu.