Fair enough.
I did a little more testing, only to realise I was editing the original ability and not the custom one... (-_-)
I also did a bit of research and found a couple of old threads that had the same issue. Unfortunately there doesn't seem to be a solution.
Editing Stampede: Blood Splashes
Object Editor Stampede spell edit
Looks like
@DoomBlade was right; editing the Art - Special of the original works as intended but any custom abilities seem to only play the default explosion, even if you set the Art - Special to nothing.
You can either edit the original or trigger the spell completely.