They will not offer a choice for sure, but they can sell a different SKU with Andoid (based on the same hardware). MS would have nothing to say, and at the end of the day I don't really think that HTC cares a about how happy MS is.
But that is just speculation.
Well, at least these latest WM devices lost the dedicated Strat, PIE, Messaging and OK buttons, which were included probably in the reference WM phone design.
Now the phones look a lot more OS agnostic, and MS has to live with that, since their os is crap and manufacturers need to fight the iPhone.