But hugger, you're Something Extra and Balsa Nova are both mid wing models. Aspect ratio or not we've already determined some time back that they'll fly nicely without dihedral because of this wing location.

Getting back to the flipped fuselage Stik that he wants to build I'd say that it'll work fine if done as mentioned but it won't be perfect. Remember the old Top Flite Contender? That one had a rep for doing all the bad things mentioned in terms of adverse roll from rudder application. But it only bothered a few folks. Those that it did bother either added a pinch of dihedral or, as I recall, modified the wing tips to give them some upward angle so the wing acted like it had some tip dihedral. That would be a definite option for this case. It would maintain the simplicity of building a one panel "slab" wing yet provide the hint of dihedral like roll coupling. I'm thinking that if the tips were angled down it would have the same anhedral effect for the shoulder wing version. I'd just go with the stock wing and try it as the low wing conversion. If it turns out that the model displays some issues with rudder induced roll coupling to the point that it is annoying then hacking off the tips and angling them up would be far less trouble than cutting into the wing's center section and attemping to arrange for some dihedral. I'm assuming here that it comes with a one piece wing in the box. If it's a two piece setup that you join then adding a touch of dihedral would be quite simple. Just a new joiner with a "kink" in the middle and an angled wedge of sheet balsa as a gap filler.

Also daRock mentioned above about how just flipping the fuselage would raise the fin and rudder area and how this would increase the rolling side effect. That's an excellent point and suggests that the fuselage should stay in the original configuration. To convert to a low wing configuration just make a new cutout in the sides and bottom which would then be glued onto the upper wing mount area. That would maintain the fin and rudder area being low and fairly centered around the thrust line and go a long way to reducing the adverse rolling effect you'd get when flying knife edge.