ssh + sudo + su into login shell
This line should works for you
ssh -t me@machine "sudo su - specialuser"
This solution give me a prompt or not depending on -t switch
ssh -t me@machine "/bin/bash -l" # Give me a prompt
ssh me@machine "/bin/bash -l" # Give me NO prompt
ssh me@machine # Give me NO prompt
Notes from man ssh
-t
Force pseudo-tty allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services. Multiple -t options force tty allocation, even if ssh has no local tty.