Hi there .... The answer to your predicament is a lot simpler than you think....

Set the 6exa to have flapperon mixing (channels 1 and 6 as per normal). Get it working exactly like you had it when the ailerons would go up instead of down like you want. Then, "engage" the flaps, so that the ailerons are as high up as they go (in the wrong direction). Then, unscrew the servo-horns from the servo, and rotate them so that the ailerons are back to neutral. Now, when you turn the flapperon knob, the flaps go down ......

Basically, with the flaperon knob on the 6EXA it is non-directional. YOu turn it in whatever direction makes sense for your plane.

gus