There is currently no way to do a lookup when season is non-numerical or zero. There is a workaround - temporarily change the season number from 0 to a number, then do an individual Get Movie and TV Info, and in the search bar change the season to 0. It should find the correct special.
In my library, I have specials as [Season]=S and extras as [Season]=X.
It would be nice if MC would do a lookup of [Season]=0 for anything with an empty or non-numerical season value. Hopefully this is something the developers might consider adding in.