They don't have the mutes in them
AFAIK.
I just put the foam under the strings and then the cover over the whole thing without any foam on top of the strings themselves.
Great thump and no after-notes to mess you up.
I also tend to use a little less foam under the G string though as it won't sing well if you use the same amount of foam as the other strings.
Retrofitting would be an unnecessary added attraction as I am sure that everyone puts more or less foam on the strings to suit themselves and one size will not fit all I think.
I don't feel this could be a niche market item at all.