Why not include header in the username field :)

Take a look at this repo: https://github.com/mrhaoxx/OpenNG

It allows you to connect multiple hosts using the same IP, for example:

ssh alice+hostA@example.com -> hostA

ssh alice+hostB@example.com -> hostB

I think that would work just fine for most use cases, though you may run into people trying to set up weird usernames on their VMs that conflict with the host split config.

Still, this is the best zero-config solution in my opinion, much simpler than the solution they decided to go with.