What will be required to transpile OCaml to Lua?

Learning
10

For the Neovim-specific use case, you may want to take a look at vcaml, which lets you write OCaml programs that interact with Neovim over msgpack RPC. Do note though that while the library as-is should provide you with the functionality you need, it is under active development so the API may change (improve) in significant ways between releases.