My approximation of the RotJ style:
Style: Default,Trade Gothic LT Std Bold,50,&H00FFFFFF,&H000000FF,&H00FFFFFF,&H00000000,0,0,0,0,105,100,-1.0,0,1,0.5,3.5,2,16,16,90,1
Use {\blur1} on each line of dialogue.
It will come out looking like this:
(Note: The fake subtitle is generated with a margin of 32 instead of 90, so that it can be seen separately from the original subtitle.)