First, you need to set up the aileron servos oer the DX7's instructions for flaperons. (Page 45 of the manual.)

Left Aileron Servo in AUX1, Right Aileron Servo in AILE
In the Wing Type menu, turn FLAPERON ; ON and in INPUT SELECT move to FLAP TRIM and set it to "ACT"

Now :

Go to the "FLAP SYS." menu, You'll have 3 positions : NORM, MID, and LAND. On the 3-position switch labeled "FLAP MIX", 0=NORM, 1=MID, and 2=LAND. Use the select button to move the cursor to the "FLAP" setting for whichever position you want to be your "slowdown position" (I chose 2/LAND), then increase/decrease it (depending on whether servos are reversed) to 125% (full throw) in whatever direction necessary for both ailerons to deflect up. Then use "select' to move to the ELEV side, and do the same. In my case FLAP is "DN125%" and ELEV is "UP200" for LAND, and 0's everywhere else.

Ta da