I have no experience with coir, but I can vouch for bone meal as a fert for seedlings, at least, here where it's cold it seems to work fine. As Al said, mixed into the bottom 80% or so is best, just because I find that fert on the top tends to promote wierd fungal stuff to grow.
The bone meal here is 4-12-0 iirc so some N as well as P. IMO the high P helps the cold situation by ensuring soluble P that's easier to take up in the circumstances. But liquid fish fert would provide that too, if watering works for you. I can't really water on a schedule here because I don't control the temperature and rate of uptake, and I want to keep the seedlings on the dry side not cold and wet.
It's possible that there's a better more balanced dry fert available with some K in it as well, but I don't know it or haven't tried.
Oh, and as regards the timing - I don't use ferts on the heat mat when germinating, but I put a few grains of bone meal into the cell pack and pot up the seedlings in 9 cells until they get a couple sets of true leaves. I've never seen any adverse effect of giving them a little fert before they have true leaves, but I wouldn't want to do that for germination stage as it might promote rots in the hot wet environment.