What we suggest with people who are dead is just to add the phrase, "If rebirth is true," at the beginning of the wish. Then use the same Compassionate/Lovingkindness wish, "If rebirth is true, may so-and-so, etc., find peace of mind"
"Does this benefit them in any way?" Some people believe so, if they're in a realm where they may know what you're thinking. I don't know. It certainly benefits you. There is a belief in Thailand that you can make merit to benefit a person in another realm, so I would imagine that generating good feelings towards them may also have a similar effect, but I wouldn't know that for sure. I don't have the powers of a Buddha to know that. There is a teaching in the Buddhist scriptures that if we develop psychic powers, a very strong concentrated mind, then we may be able to go to those other realms and teach them, but it was usually the Buddha who was doing this.