Did anyone ever get this working? I have a similar request and opened a new thread but just stumbled across this one but see there was no conclusion what works.
It also did not make sense why you would create a "guest" user account on FreeNAS as it requires a password which defeats the purpose of a guest account.
It works. It doesn't matter that you are setting a password for the 'guest' user account because Samba will automatically map "bad users" to the guest account and not require credentials.
Consider the following situation:
You have a freenas server with the share \\freenas\share and the guest account "guest" (which in the windows world would be described as \\freenas\guest). You normally log into your laptop with the account \\laptop\bob. When you first try to access "\\freenas\share", your laptop will first try to authenticate using "\\laptop\bob". Since the user account \\laptop\bob does not exist on the FreeNAS server, and you've checked the box "allow guest access", freenas will auto-magically authenticate you as \\freenas\guest. You will now have whatever access \\freenas\guest has to the share \\freenas\share. If "guest" owns the dataset / share in question, then you will have the "full control" permissions set on the share.
In the above example, you may run into problems (and people often do), if you have also have a user account "bob" on the freenas server. Freenas will evaluate the credentials being passed to it and determine that the user "bob" is not a "bad user" and not do the auto-magic user mapping. This is one of the most common causes of problems with "guest" access.
Another common problem is that people want guest access to \\freenas\guest_share, but regular authenticated access to \\freenas\share. They will authenticate to \\freenas\share using the \\freenas\user credentials. Then expect to be able to get the same level of access that guests do to \\freenas\guest_share. This is not necessarily the case. Windows only allows one set of credentials to be used per windows (or samba) file server. In this situation you will need to configure permissions on \\freenas\guest_share to grant access for \\freenas\user.
TL;DR,
It works if you configure it correctly.