First off - yes, having a default homepage generally is considered to defeat the purpose of Incognito mode... however I work in a public library and we're setting all our public computers to default private mode to help cut down on patron credentials being accessible by the next person using the computer (particularly after the latest google login "improvement"...)
Most of the search results I get on this boil down to "that defeats the point" or "you can't do it."
Here's the thing. I KNOW there's a way for it to be done, because out of our five public computers, three of them open in incognito mode with a homepage displayed, and two display the regular 'you're in incognito mode' message. As far as I know I installed Chrome the same on all of them, and so I'm left particularly confused by this.