JOEL - I don't think that any General Authority of the Church has specifically addressed this question. It is commonly accepted in the Christian world that, normally, one would not pray for someone who has already passed on. However, I can think of a few exceptions. For example, A person might pray that their deceased non-LDS spouse will accept the gospel in the next life so they can eventually be sealed together forever.
Also, I don't think there is any way you can stop a young couple from praying for their baby that has just died, even though they know that baby is guaranteed to end up in the Celestial Kingdom of Heaven. This one I know about from personal experience.
Mostly, I think prayers such as these for those who have passed on provide more comfort and help for the one saying the prayer, rather than for the subject of the prayer. Personally, I see nothing wrong with it in some circumstances. I am sure God understands why you are doing it. Fasting and praying is an expression of your faith in God and what you believe He can do. I know He is always willing to listen and help us with whatever our righteous desires may be.