I think that the Miracle Man neck, though probably pretty smooth, will inevitably be a lot more compressed than a Mule! You could probably put a Mule in the neck, but you might have to mess with the pickup heights to help balance the relative output differences. Alternatively, you could go for the Abraxas (like a "Mule+"), which would keep up with the MM bridge better than the Mule, but still be pretty open compared to the MM neck.
Have you checked out the Miracle Man clips in the Players' section? I'm sure II remember some people using the neck for bluesier takes, and that may help you get an idea. And, of course, check with Tim :)