I played a gig at the Cheddar folk club in Somerset about 15 years ago and the residents were a husband and wife duo called Macanabba.
She had a dulcimer with a removable 6 1/2 fret. The fret was mounted on a piece of dowel which fitted into a horizontal hole drilled from side to side of the fingerboard with a gap on top. When you didn't need the extra fret you took the dowel out and replaced it "upside down" and the unfretted side of the dowel filled the gap in the fingerboard.
I personally usually use DAD tuning with a capo and have no problem using the 6 1/2 fret.

Grahame Hood, Bromley Kent