When I was seriously contemplating playing 5 string banjo a good number of years ago I also wanted to use it to accompany songs and thus I wanted to play in keys where the "G" drone of the 5th string would have been wrong. So I needed a "capo" for the 5th string in addition to that that I used for strings 1 to 4 to play in, say, the key of Bflat. I found such a device and it worked but in order for it to do so I had to screw a bar onto the side of the neck adjacent to the 5th string. There was a small right angled damper that slid along this bar and fretted only the 5th string at the fret of need.
So to play in, say, B flat the main capo went on at the third fret whilst this 5th string device was applied at the 8th fret. In this way the relationship between the strings was maintained.
It worked but the downside were 2 horrible screw holes in the neck of my banjo.
Although it worked I didn't master the techniques of playing banjo well enough. One might say I frailed miserabely!
Good luck