SMF has a ton of support for bridges etc on their support site but I don't think they'll fit so well. The main problem is people already have accounts and allowing people to login to existing accounts can't happen without a setting change.. I am also not sure this scheme will readily fit within the confines of a normal plugin. I will have to make a custom query for the alternative login.
My solution - Add a new text field to each user labelled something like "Bitshares X allowed login name"
This will be blank for existing accounts by default meaning someone can not use Bitshares X to login to that account. Once the user changes this to a registered name then the owner of that registered name will be able to login to SMF.
Likewise, if someone tries to login via Bitshares X to a non-existant name then they will be sent to the new user page. The idea is to force them to create an account with some sort of password so they can login later. The other option is to skip the new user creation page, but then they'll never be able to login outside of Bitshares X without creating a password. (And I am not sure how much code is required to have accounts with no passwords.)
So this allows existing accounts to start using BitShares X and BitShares X users to create accounts.
The other main option is to just have a "allow BitShares X login" checkbox, but it isn't near as flexible.
Thoughts on this ? Concerns? I'm not sure if anyone will read it, but hopefully Toast/Bytemaster/the XTS authenticator guy might.