I imagine Jackson will be busy with Hobbit
-related work, whether formal post-production or publicity, until the last film comes out in December 2014. So that would rule out his working on series 8, if the reports are accurate about that filming from October 2013 to August 2014. But we can rule that out anyway: directors for the upcoming filming will be in place already, not at the "I think it's entirely possible" stage. If the annual production schedule is back on track and series 9 starts in October 2014, Jackson could plausibly be asked to direct something from the second half of that series. That's assuming Moffat is correct in judging that he's serious about it, and that any other wrinkles could be worked out. Even if Jackson genuinely wants to do Doctor Who
, he's got a bunch of other film projects in development that might have to take precedence.