The Looms have the advantage of being canonically manipulatable, after all.
A sufficiently motivated Doctor could probably will the process in a particular direction, perhaps even unconsciously. We've already seen him pick up things in the regeneration process -- though goodness knows what the showrunners really think of how the "half human" bit got worked into it all! Heck, the first time we saw it, it regenerated his clothes too!
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The Looms have the advantage of being canonically manipulatable, after all.
A sufficiently motivated Doctor could probably will the process in a particular direction, perhaps even unconsciously. We've already seen him pick up things in the regeneration process -- though goodness knows what the showrunners really think of how the "half human" bit got worked into it all! Heck, the first time we saw it, it regenerated his clothes too!