We'll agree to disagree on that one, then.
I see the Master as smart enough to accept it was possible to begin with.
Another fan theory goes that now that all the other Time Lords are dead, the Doctor could potentially be the recipient of all that regeneration energy and thus could afford to wile it away on stuff like growing the handyclone or fixing River's hand. Of course, we'll likely never really know how regeneration actually works, though the extended universe of Doctor Who has a loooooot of factoids that may or may not contravene what we see on screen:
http://tardis.wikia.com/wiki/Regeneration
Mark