Neovim 0.2+ adds a Qt based editor in order to hit its promise of "works on Windows". It should be possible to leverage this into an alternative editor pane, all that would be needed is to channel each node's body text into and out of Neovim, (the in-portion would also require you to set filetype to correspond to the "in-force" @language directive).
Why I say this should be easy is that Neovim uses Msgpack rpc to give you complete control of Neovim. Three things are needed:
1) provide a way to embed the Qt frontend as a new/alternative editor pane and connect to an running neovim instance.
2) switch all the leo code that fills/updates the current editor pane into msgpack calls that fill or update the body text of the current node to/from the neovim pane.
3) pass back to leo any calls that change the Outline pane, e.g. alt-downarrow.
I know that Edward prefers emacs over vim, but just as their are vim emulating modes for emacs, there are emacs emulating modes for vim.
(sorry about the partial topic above)
Tom