what's the syntax for Accessing smb/windows shares via alternative ports?

Can't be done sorry. Windows does not support TCP ports other than 445/139. (Here is one reference.)

You could try a Windows port of rinetd on the client machine, which would let you do something like this on the client machine:

#bindaddress  bindport connectaddress connectport
192.168.1.1   139      4.2.2.2        999

Assuming that worked, you could use your normal \servername.server.com\sharename from your client machine, and rinetd would change the connection port for that address.

This article: http://codewut.de/content/port-redirection-windows has more information and a patched binary. I haven't tried this myself, but it looks like it is worth attempting.