Seems like this is not supported ATM. There's a dedicated ssh_key key you can use when setting up the ssh connector (documented here), so something like this:

hosts = [
    ("my-host.net", {"ssh_key": "~/path/to/key"}),
]

But that would be used for the entire execution for that host, not just for the ssh.command() operation :(

i end up with custom bash script, rather than using ssh.command feature