At Haveyoubeenhere we have the centralized platform, where in a way you could say everyone is a guest, we currently don't mix comments from the centralized platform with those crossposted to the Steem-blockchain.

One of the immediate challenges I can see pop-up in my case is that not every user will exist on both Steem and Haveyoubeenhere so @username would be strange if the user exists on Steem but not on Haveyoubeenhere.