Depends on the context of the story(s), I suspect. Considering he only made the one appearance, they may want to stay pretty close to it for those who only know of him from a couple shots in the show and maybe looking him up online. Changing his appearance drastically (if it is a one-off and not a series of 'webisodes') kind of undercuts bringing him back at all.
First picture I get from Google image search with "paul mcgann doctor" is this one: http://onceuponageek.com/images/doctorwho_mcgann2a.jpg
So they might as well use that look or some variation of it.