You can; if you supply a small number, it takes it as a screen number and adds 5900 for the port number, but if you enter a large number, it takes it as a port number. (I don't know where the cutoff is, but it should work fine unless you're using a rather low port number.) But what I do is ssh -L5901:127.0.0.1:5900 ... so I can connect to localhost:1, leaving :0 open for a VNC server on the tablet.