As far as I know, the longneck was an invention of Pete Seeger. He, of course, a player with considerable vertuosity. I suppose the one Seeger owned was custom made for him. You can see it if you look at a "Rainbow Quest" video. I've never seen him capo below the tuner for the fifth string and I can't see how a capo on the third fret would interfere with the fifth string.

Five string banjos here in Australia are rare. I envy you Uncle Jaque for finding one so cheap.

Murray