This seems to be a serious possibility.
According to this Radio Times article (July 2014; emphasis mine), Moffat and Jackson were seriously considering this idea, although there were doubts whether Jackson's schedule would allow it, and they weren't sure where it might be filmed (New Zealand or Cardiff):
Doctor Who showrunner Steven Moffat has confirmed that the prospect of Lord of the Rings director Peter Jackson helming an episode is still very much alive, and that talks are ongoing.
Doctor Who fan Jackson, who cast the seventh Doctor Sylvester McCoy in the fantasy trilogy, told journalists last year that he’d be, “very happy to” direct an episode and that discussions were “actually kind of serious.”
Moffat, though, says that, while he’s still open to the idea, it all depends on the director’s schedule.
“He’s still incredibly busy on The Hobbit,” said Moffat. “I’ve spoken to him face to face, and he would like to do one. He accepts that there’s no money and that there’s no time, and it would have to be when he’s available – and I don’t think he’s even been available enough to answer our emails of late! I think it will probably happen at some point.”
The director, notorious for making long, bladder-busting films, has also said that he would relish the challenge of the small screen.
"I'd love to try my hand at television, because I've never had the discipline of having to shoot for those impossibly tiny schedules. I think I could do it okay now."
According to this later Radio Times article (November 2015):
Doctor Who showrunner Steven Moffat has wanted to get fellow Who fan (and blockbuster director) Peter Jackson directing an episode of the series for a while – and while in New Zealand for the Doctor Who festival, Peter Capaldi apparently took matters into his own hands.
In a new sketch, Capaldi’s Twelfth Doctor invades the home of the Lord of the Rings director to try and get him to sign a contract with the BBC – only for Jackson to get distracted by polishing his Oscars and watching The Walking Dead.
Jackson put up an video of the incident with Capaldi on his Facebook page:
Do watch it; it's hilarious!