Following on from my previous post I was chatting with a friend and they pointed out that I could also make use of preexec to get my desired behaviour.
So I removed the entries for wide links and unix extensions from my smb.conf, allowing them to go back to being the default. I then changed my mythtv share to contain:
[mythtv] comment = Myth TV Recordings path = /auto/mythtv guest ok = Yes preexec = ls /auto/mythtv
This then solves the issue of Samba not allowing the share to be opened due to the "canonicalize_connect_path failed for service " error.