I started building my own version of a hybrid, Banjo/Mando inst. a couple of years ago and so far I have built 6.
I jokingly call it a "Manjo"
Many years ago Rick Fielding and I talked about the possibility of such an inst. but I never followed up on it till now.

I use a 5 string banjo scale(complete with 5th string, played Scruggs Style) mounted to a large octave mando body. The sound is surprisingly sweet, yet has the fullness of a parlour guitar and is more than adequate to accompany vocals.
Dont have a photo up on my webpage yet, but if you email me I can send a photo jpeg.
Check out the octave mando photo on the website and imagine it with a banjo neck.
cheers, Glen   www.onlink.net/~glenreid/index.html
glenreid@onlink.net