Ah, I see, that's a different thing than I've been doing. I have a bridge for Cocoa that lets Lua post and observe NSNotifications, and so I have a stub class in ObjC that can call back into Lua in response to receiving a notification. I'm not using FFI at all.
The way I do it is, there's a C function that takes a Lua function as an argument (through the stack) and sticks it in the Lua registry, then stores the (int) index that it's stored at. Later on I can pull the function back out into the Lua stack and pcall it.
I'm still writing it, but my plan is, have all communication between Lua and everything else go through NSNotifications, to enforce the idea that the game logic (I'm writing a game) should be in Lua and the animations / etc. in ObjC. If you'd like to look at the bridge, it's here:
The way I do it is, there's a C function that takes a Lua function as an argument (through the stack) and sticks it in the Lua registry, then stores the (int) index that it's stored at. Later on I can pull the function back out into the Lua stack and pcall it.