If I conquer with Last Rites, can I hide a unit directly from the trash?

No, you cannot hide a unit directly from your trash.

According to RiftJudge FAQ #9774, "hiding" is a specific game action (Rule 421) that requires the card to be in your hand or your Champion Zone (Rule 421.2.a). The effect of Last Rites only grants permission to play a unit from your trash, which is a distinct game action from hiding. Because your trash is a non-board zone, it does not satisfy the requirements to perform the "Hide" action.