I just wanted to say, regrettably, this is the solution I've taken.
Looking at ash-tray, a helper wrapper around ash(from git master) and winit. When adding imgui I discovered that I could use the preexisting vkr-imgui. I needed to patch vkr to support the loading of ash primitives.
Ash-tray uses submodules and scripts to bring in usable versions of other crates. I couldn't find another way.
Of particular note is that the root of the crate in both ash and vkr is not the same as the base of the git tree and currently cargo doesn't support git subfolders.