As you said, the Slingbox M1 and Slingbox M2 does not have the option for guest access.
To answer your question about the Slingbox 500, yes they support the guest access feature. They were the last unit with the ability to use this feature. Below is a link that will explain how this you can enable guest access for the Slingbox 500 (and other models as well):
I hope I have answered question!
Sling Media Moderation Team