let me just say, although I dont have tons of experience, Using a one piece flatsawn sounds like a bad Idea.
if the piece isnt even recommended for regular guitars, I dont see the extra length and tension of the bass strings making it much better. Regardless of how wide it is.
Also, depending on the flatsawn piece and material, it most likely WONT have Quartersawn characteristics on the edges. I have plenty of maple, Mahogany, paduak, ETC that is 6"+ and caries the same ring orientation. Unless your much closer to the heartwood, I dont see the grain doing that. also, if It did, Id say the piece was more likely to warp under tension.
I think that if looking for a one piece, a quartersawn would be the best Idea.
But your less likely to find that at a regular lumber yard.
what I would do, is get some hard flamed maple pieces(looks pretty) and cut the pieces so the grain orientation differs.