How to prevent nuts and fruits from sinking to the bottom of a cake during baking? Especially slow baking
Try coating them with flour before adding them to the batter. I've heard it helps prevent sinking.
Yes, toss them with 1 tbsp or so of flour before adding to the batter.
If you have a relatively thin batter, fruits and nuts will sink because they are simply more dense than the batter. You'll have better luck adding them when you are using a denser batter. I've not had a lot of luck with flouring them before-hand, unfortunately.