You don't need to be arrested nor even charged in order to be pardoned.

You also don't need to be specifically identified in order to be pardoned.

For example, all American citizens who dodged the draft were pardoned at once - proactively before they had been identified or charged. It allowed those who escaped to Canada to return home and all those who could be gone after in the future to rest easy.

Trump can submit a blanket pardon for "all those who were provoked by the theft of my landslide victory - everyone knows it - into engaging in a mostly peaceful protest in their capitol and may be retaliated against by the crazy and nasty left in the future".