This is exactly what I do with Decorum, yes. This sort of thing can be done because the programmer can decide from which values the address of an SD object will be derived. The address of an SD object is SHA-512(tag_type + identifier), and we pick the tag_type and the identifier ourselves. By establishing rules (a protocol) how these are chosen, the addresses become predictable.
For example, the decentralised forum of Decorum will allow anyone to create a new forum category (think subreddit). I establish the rule (this is an example, it's not accurate) in the protocol that every category SD object shall have tag_type "12345", and as identifier it shall have the SHA-512 of the name of the category. Let's say someone wants to add a category "Music", then the address of that SD object = SHA512("12345" + SHA512("Music")).
Yes, that's the core feature really!
On SAFE we won't integrate third party plugins like "Facebook Comments". Everything is already integrated with the SAFE Network itself by definition, so everything can be embedded/linked/shared between websites. Websites/apps become merely user interfaces and protocol implementations to interact with the data stored on this huge global unified filesystem, the SAFE Network.