I certainly do not have good or complete answers for all these scenarios at this time but I can say:
1. RoR question: we currently provide a mechanism for authenticating against and external system as well as sync the user attributes with the source. We currently have an LDAP auth plugin doing something similar. They would essentially have to write a plugin to communicate with their auth system. The plugin required is lightweight.
2. attribs in the token table: this just sounds like a bad idea. We really should not be mixing data not related to the survey system in the survey tables. I can see why someone would want this type of functionality but we need a better strategy for handling there cases.
3. Adding fields to registration: this seems to indicate people want to collect more data as part of a users profile. We can account for this by allowing admins to customize the user profile page
4. Competing profile before continuing survey: This is an interesting problem. What indicates a profile is complete? The minimum we currently require is a username and password. I do not think we want to get into the game of requiring much more as we want it to be a light weight process for users. Perhaps as part of #3 we can also allow admins to set the required fields. As for requiring the profile be complete before a survey can be taken... I can see this being a type of survey level conditional but we have not put much thought into that yet.
5. More than two custom attributes in tokens: I think somewhere in my above comments this is addressed.
I hope this helps. Now just an FYI this message by no means is an indication that any of this will be available anytime soon. We need to get the core of the system solid before we begin to worry about these types of additional features. If you get a chance it would be worth adding some of these to the Mantis Roadmap for 2.0 Beta.