Its set up as a new row in vfx_persistent.2da, you just use a point emitting the fire effect ( or whatever effect you want ) and specify that in the "FillSEF" column, and the game distributes this thru the wall effect. Then you implement it via using that row number when you do the AOE via scripting. ( i set up alternative SEF's and the like for walls of every element type ).
example newly added wall spell which does acid instead of fire
ROW = 101
LABEL = VFX_PER_WALLACID
SHAPE = R
RADIUS = **** <= not used, R means rectangle
WIDTH = 10
LENGTH = 2
ONENTER = SP_wallfireA <= my modded on enter script, i am using the wall of fire script
ONEXIT = ****
HEARTBEAT = SP_wallfireC <= my modded on heartbeat script, i am using the wall of fire script
OrientWithGround = 0
DropToGround = 1
VisualFX = 2180 <= sef reference to visualeffects.2da, you can use row number or sef name, this does the light for it since if it's in the fill that is a lot of lights
SoundImpact = ****
SoundDuration = ****
SoundCessation = ****
FillSEF = sp_wall_of_acid.sef <= sef file i created
SEFSize = 1 <= scaling amount which can adjust the sizes of the visual effect, use to make sure they match the size exactly of the radius
Then i use 101 in the function to create the visual effect which refers to the correct number, and i make sure any damage done matches the visual. ( i do visuals when targets are damaged each round to match, and do the damage of that given type, all of this is done via scripting )