
I would say it can be done with fiddling. I'd like to take a peek at the schematics myself. unless they are doing what toyota do with their video (IE digital) then I believe its doable.
the nissan GTR (local) is a clsoed system. didnt stop me from shoving two cameras and a external navi into it though.

Keen to see if anyone has been able to do this yet?

When I was at the Mazda Museum in Hiroshima last April I spotted the factory reverse camera. I think they have realeased it as a Navi pack. Link below.
【MAZDA】アクセラ Navi Edition (特別仕様車)|カーラインナップ
While cruising the interweb, spotted these. Looks tidy, but not subtle.
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GENII 2009 VR MPS LUX
Boss: Okay, I mean, we can see your bulge.
Dale: Okay, first of all, they're extremely comfortable. And second of all, at this point in your life, if you haven't seen a bulge, well, I just feel sorry for you.
(http://www.hark.com/clips/wmrwmhmtsn...see-your-bulge)