Unfortunately, doing what you want is not possible with samba.
Setting ready only or writeable in samba is on the share. So if you set a share as read only, it will be ready only no matter what interface.
You can specify a write list, for users that have write permission. Again that would be on the whole share.
I maybe wrong, but that is how I understand it, based on my reading of the Samba man pages. http://us1.samba.org/samba/docs/man/manpages-3/