I have checked a few idiom places and general google, but found only usages of this construct, not any explanations.
Is this a silly corruption of "Who's keen", or is there something more interesting going on here?
Does it express anything other than "Who's keen" informally?
P.S. Check out the occurrences on the internet - doesn't really look like "how keen are you", and the first few instances occur in otherwise more or less well spelled contexts. In fact, it looks a bit like how "ask" turns into "axe" with some people...