But he painted Mat wearing the foxhead medallion on the TFoH cover (then again, I think it's not seen on the CoT cover, but at least Mat wears his scarf there...).

Anyway, as you and darius_sedai figured, the main idea is that the dull dagger & the medallion could be combined to defeat the gholam. In my view it's possible that besides in a confrontation with the gholam, the dagger will additionally later be used by Rand, when he plans to sneak into the Pit of Doom (especially if the gholam fight basically proves that the dagger works).