Would it make any sense to retune the first and fifth strings of a "guitar banjo" (which is essentially a guitar neck with a banjo sound board) and play it like a G 5-string banjo?
What's your goal? Seems like, sure, you could tune a fifth string like a drone while you save up for the real banjo, but the drone tuner is up at the fifth fret to keep it out of the way of your first-position playing.
Git-jo is about either allowing guitarists to get bluegrass sounds without learning anything, or to get the rhythm chunk of dixieland jazz, I think, and you're sounding like you're just wanting a real banjo.
It absolutely can be done, but it will definitely change the flavor of the 'banjo'. You won't have the crack and lack-of-sustain that banjos are distinctive for, but you will have something playable. You may not want to hit bluegrass tunes with it, per se, but another smoother genre of tunes (such as Old Time or Parlor tunes) would sound appropriate on it.
Foggy Mountain Breakdown would sound a bit weird, but it would technically work.
So if you're doing this to learn banjo, I say pass on the instrument grafting and buy a starter banjo. That way your ear will develop the proper banjo sounds with the proper banjo moves. If you're just thinking of tinkering, got for it.
Also: Check out Rob Bishline's Madera