I decided to forgo having markers on the front of the fretboard, but I have seen a few banjos with a star between the 4th and 5th fret and liked the look so I decided to do that.  I cut a star out of mother of pearl and routed the fretboard like I did for the peghead.  After gluing the star in place, I filled the gaps with a mixture of ebony dust and wood glue and sanded everything down.

Star and routed fretboard

Star glued in place before filling

Star after filling gaps