Mysql database in a symlink-ed directory gives ERROR 1005 (errorno: 13)
I managed to solve this, basically by the advice given by Mat. 

The first valueable tip was to use ls -Ll to show information  for the file the link references rather than for the link itself.

It is clear that the symlink should show the user (and group) mysql is using.

When mouting (in this case the cifs type) I first used 

mount -t cifs //nas01/myshare /mnt/nas -o user=nasuser 


where I should have used

mount -t cifs //nas01/myshare /mnt/nas -o user=nasuser,uid=mysql,gid=mysql


This resolved the errorno: 13 because ls -Ll now showed mysql as owner and group for the symlinked directory.