I haven't looked at the video but you want one side solid and the other side semi transparent graduating to transparent?
Use two objects shaped the same. Object 1 is ghosted with your transformation. Object 2 is only half of object 1, the side that isn't transparent and is not ghosted. Overlay them.
set display mode 800,600,32
sync on
sync rate 0
rem texture 1
adj#=255/280.0
for x=0 to 127
c=x*adj#
ink rgb(c,c,c),0
box x,0,x,127
next x
get image 1,0,0,128,128
sync
cls 0
rem texture 2
ink rgb(255,255,255),0
for x=64 to 127
box x,0,x,127
next x
get image 2,0,0,128,128
sync
make object plain 1,10,10
texture object 1,1
ghost object on 1
make object plain 2,10,10
set object 2,1,0,1
texture object 2,2
sync
wait key
Or build the level in sections, and only texture the sections you want turning invisible with appropriate textures and ghost them. Leave the rest un-ghosted.
Enjoy your day.