you say..."singularly unsuited to melody play"...and ".....then you're in the worst of all possible worlds"....
This all sounds a bit drastic - could you expand on this (I am genuinely interested - it is not just that I am hearing what I don't necessarily want to hear)? Many thanks.
Well, if you can pull it off then more power to you!
What I mean is that a long-scale instrument tuned GDAD will be too unwieldy to do a good job playing fiddle tunes on, especially reels. That doesn't mean there aren't tunes you could get around on, but you won't be able to handle every tune that comes down the pike (unless you've got monstrous hands!).
I've got an octave mandolin here with a scale length (meaning nut to bridge, essentially) of just over 24", just about the outer limit of comfort for most players playing trad melodies (long scale tenor banjos, the 19 fret models preferred by top banjo players in Ireland, generally come in at 23" by way of comparison). Tuned GDAE the high B is at the 7th fret, some 8" up the neck from the nut. That requires a stretch, a shift, or both, but is still manageable with normal-sized paws. Tune it to GDAD, however, and the High B is at the 9th fret, 10"+ from the nut. That's WAAAY up there, and I don't see how you could play, say, Jenny's chickens at session tempo while making that crazy shift.
Things get a bit worse on a longer-scale instrument. By my cocktail napkin calculation, a 26-1/2" scale bouzouki tuned GDAD would have a high B at the 9th fret some 11 inches from the nut. That's just unmanageable.
A good question would be, what are you gaining in your melody play by not using GDAE? GDAD is a drone-friendly accompaniment tuning, not optimized for fiddle tunes.