neovim supports co-processes, so a Holochain conductor could be spun up with syn running, and then some sort of IPC mechanism could support the rendering & tracking of edit commands between that process and the neovim UI (with the conductor of course synchronizing changes with the conductors of other agents editing the same document). it might be tricky to present a file-like API to neovim - I’d expect that to be a challenging part of the project.
presumably other editors could be supported too, with a modular-enough architecture.
I might dig into implementing this. making a topic in case others have had the same idea or find it interesting.