I do Caravans with my LS and all 8 players get seals.

If BLM: Use whatever your instant skill is when you see a mob pop.
If WHM: Pretty sure that just healing counts you for participation. If not, throw out a Holy on the boss mob.
If other: the boss mob lasts a long time no matter what so all 8 players can easily hit it.

Even the PLD gets full seals in my group since they have to tank the boss mob.