ssh config - do not execute remote command
Solution 1:
I'm fairly sure you can't do that and like you I can't find a way of doing it in the manner that you want. If typing ssh -N host
doesn't appeal then you could always try creating an alias if mosh supports it.
Solution 2:
I don't know about mosh, but for the OpenSSH client you can specify a ProxyCommand in the config file. The relevant entry for hostname foobar.baz.edu (my favorite university) and dynamic forwarding of port 9999 would be:
Host FooBar
HostName foobar.baz.edu
ProxyCommand ssh -ND 9999 %h