I've never actually realized that Slayer has no real way of adding lives or team life tickets apart from maybe events that may or may not exist.
An item that when touched checks if the player is in a slayer minigame, then if that minigame uses team life tickets or individual client lives, and if all conditions are met gives the player a life for the respective life system and fades out for a while like normal picked up items do.
Visually, maybe it could be Wolf3D style (a blockhead head inside a shadeless flipped normals blue cube)?